Go slow! Seduction means anticipation.
Make the anticipation build throughout the evening for both of you. This may not be easy, and you will have to be able to read your lover's body language enough to make sure anticipation doesn't turn in to frustration. Prolonging the love making for an hour or two should be fine. Prolonging it for four or five hours is probably too long. If your lover is constantly making advances and you keep pushing him or her away throughout the evening, the result will be frustration, and in some cases, that may be the very thing you are trying to overcome in your relationship by planning this evening.
The best way to make sure things last without dragging them out too long is to start the evening about an hour before dinner. Depending on whether you are going to seduce your spouse or lover that lives with you, or you each have you own place will depend on when you get the evening started.
If you are going to seduce your spouse or a live in lover, and it is a work day, plan on having dinner about an hour after both of you are home. If it's a weekend, get showered and dressed about an hour before dinner, and ask your partner to do the same. If you are inviting a lover over that doesn't live with you, have them arrive about an hour before dinner.
Once you are both dressed, or your lover has arrived, put dinner in the oven, pour two glasses of wine, turn on some soft music and keep the volume low, then sit down together and talk while dinner is cooking. Don't turn on the TV. In fact, it your partner is one of those individuals that just can't seem to survive without turning on the TV when they get home, then take the batteries out of the remote (this will usually take care of the problem for most men, though they may become mesmerized and stare at the blank screen, or beat the remote on the coffee table repeatedly trying to make it work. For women, you may have to unplug the thing, since they seem to know how to turn it on and even change the channel without the remote).
Your goal at this point should be to develop the communication skills between your partner and yourself during this time, and it can be a make-it-or-break-it situation for many people. If you're having problems in your relationship, don't talk about them at this time. Direct the conversation around the fact you love one another. Compliment your partner's clothing and appearance, and ask your partner how he or she likes the outfit you are wearing. Tell them how much you love them, and how you have been looking forward to this evening.
If your lover is the type of person that complains constantly about how much they hate their job, and can drag on for hours telling you about all the horrible details of their day at work, then don't ask how their day was. In fact, if you notice them starting to talk about it, try to be subtle and lead the conversation in a different direction. Since only you know your lover, you will have to decide what to talk about, and what to avoid.
Don't use props at this time. Jumping up to go get something you bought and just can't wait to show off will draw their attention away from you. So will grabbing a magazine to show your lover a picture of the latest gadget you want or to have them read an article you liked. You want to focus your attention on each other, not the rest of the world.
While talking, be seductive. Turn sideways to face your lover. Smile a lot, & touch your lover frequently. I'm not talking about heavy petting here, so keep your hands under control. Use your finger tips more than the palms of your hands. Gentle touches to the face, arms, legs, and hands are very effective.
Communication seems be a major hurdle in many relationships, so if communicating is a problem, and you feel tension building, try adding some levity to the conversation. A little light hearted laughter may ease the tension.
If talking is simply not working, and the conversation runs dry, don't force it . Try slow dancing (thus the reason to have the soft music already playing). I know some people aren't good at dancing , but a slow dance can just be holding one another and swaying back and forth to the music (I have two left feet myself, but even I can pull that much off).
Know yourself, and know your partner.
It's important to know what type of person you are, and what type of person your partner is. Many people still have a lot of sexual hang ups, both men and women, and if you're one of them, you need to get over it because you're missing out on a lot of fun having sex. If dressing up and acting sexy embarrass you, makes you nervous, or causes you any other negative feelings, you need to realize that the things you and your lover do together are private, and the world isn't watching you perform.
Unless you know your partner intimately, don't try surprise, it usually backfires.
Surprise can be very effective to create an air of mystery and anticipation. It can also backfire. Unfortunately, the latter is usually the case. Watching couples on TV and in the movies surprise their lover with exotic evenings seems like a great plan, but the movies aren't real life, and the characters the actors are playing aren't real people in real life situations. You don't get to 'cut' the scene and start over if something goes wrong, so unless you know your partner very well, and know for sure they will react in a positive way, when planning your first evening, don't try to make it a surprise. 
If you're into B&D (Bondage & Discipline), and you're with a timid woman, showing her your collection of whips and chains and telling her you want to tie her to the bed and make her tell you how naughty she is so you can spank her may make her open up and try it, or it may send her running. This may actually send some men running too.
Along those same lines, if you're into threesomes, and you invite a third party to show up unexpectedly, you may get lucky... or you may get slapped and dumped on the spot.
The point I'm trying to make here is that you need to know who you are, who your partner is, and know how to handle the evening to keep it from turning into a disaster for both of you. If you and your partner are not compatible when it comes to sexual experimentation, then both of you are going to have to be willing to compromise on different occasions to fulfill each others desires. Refusing to do the things one another enjoys can be devastating to a relationship.
Communicate your intentions for the evening.
Tell your lover in advance that the two of you are going to have a romantic dinner on Friday night, or whenever. Let them know you want them to dress nice, and that there will be candle light and soft music, and that afterwards you would like to spend the evening talking, making love, and trying new things. If you're going to try something a little exotic, such as bondage and discipline for the first time, tell them in advance. Tell them that you have bought a blindfold and some restraints, and after dinner you are going to tie him/her up and play out some fantasies. Let them know you're goal is to create an evening together like you've never had before, or haven't had in a long time.
You don't have to give everything away, but let them know what to expect so that you don't get rejected. Being told 'No' half way into your evening can, and usually does, destroy the mood, and sometimes the relationship. I use B&D a lot as an example because it's one of the most popular ways to explore new sexual experiences, it's an area that many people want to experiment with, and it's an area that many people are afraid of. A lot of trust is required to let someone physically restrain you, especially if you're blindfolded.

Making preparations.
Candle light and soft music are always romantic, but don't stop there. A candle lit dinner with soft music in the background is the obvious choice for a romantic evening, and it's always good for setting the mood, but we're going to take it a step farther. Don't make it boring, especially if you're married or in a mature relationship. Just adding candles to the table isn't going to make this meal any different than any other meal. You're either trying to create some new emotions in your relationship, or revive some emotions that have subsided over time. You can't do that effectively if the surroundings are the same as every other night. Get some new things that your partner has never seen before, such as a new tablecloth, new place settings, new silverware, & new wine glasses. Change the decor in the dining room by getting some new paintings or curtains. Do the same in the living room, den, bedroom, or whatever rooms of the house you are going to be using for the rest of the night. Moving the furniture around and adding a few new pictures or accessories will have a positive effect by creating the illusion of new surroundings, eliminating some of the complacency we discussed earlier. Remember, you're goal is to create an evening together like you've never had before, or haven't had in a long time.
If you're not offended by erotic images, then look for some paintings or statues of nudes, especially couples embracing, to help set the mood for the evening. I realize that some people look at all nudity as pornography, even the statue of Venus or David, so you will have to go with what you feel comfortable with, but unless you have a religious or moral objection to erotic images, try to expand your perceptions. Remember, the whole world isn't going to be looking in your window to see what's on your walls, and if displaying erotic art embarrasses you, you can always put the paintings and sculptures away when your mother or mother-in-law comes to visit (or before the kids come back from the baby by sitter's).
Dress for sex.
You may have a wardrobe full of clothes and can create different outfits for different evenings, but unless your lover has never seen what you have hanging in your closet, and you already have some very erotic outfits, don't just go through your clothes looking for something sexy. Get something new for tonight. Just like the dinner setting, you don't want your partner to see the same thing he or she has seen before. Even if you already have stockings and a garter belt, if your lover has already seen them, get some new ones in different colors or with different styles.
There are dozens of different styles of erotic underwear for both men and women. Buy a variety in advance, then you can mix and match them on different occasions. After all, this evening should only be the first of many erotic nights you and your lover share.

Get your partner to dress for sex as well.
Why not buy your partner their clothing an underwear? This may seem like an odd suggestion to many people, but since buying clothes is usually a somewhat personal experience, it will actually help bring you and your lover closer. Just be careful if you try to buy them as a surprise. As I said before, surprise does not always have the desired results, and unless your partner knows in advance that what you will be buying is of an erotic nature, he or she may refuse to wear it once the packages arrive.
If you're not sure how to approach this idea with your lover, the best way is to bring it up after you have suggested a romantic evening together. Once you receive a positive response, tell your lover that want to create a night of intimacy, passion, and try some new things. When they ask 'Like What?' continue and explain that you want it to be a very erotic experience for both of you, and that you would like to buy both of you something new and erotic for the evening.
Once your lover agrees, use the internet to your advantage rather than making time to run around to different clothing stores. The idea of you buying your lover's clothing may seem fine at home, but once the two of you are actually at the store, one or both of you may begin to feel uncomfortable, especially when you start shopping for erotic underwear, which is usually not available at most stores, and will require you to visit an adult shop. It's much easier, and more comfortable to shop from your home, and you can visit adult toy sites without the risk of embarrassment. More...