|His First time !
In the both-first-timers case, I recommend strong egos, mutual respect and affection, and a great sense of humor. Theoretical knowledge also helps some. What follows is a list of guidelines you should follow with him to make the experience go as smoothly as possible. It is also written on the assumption that he is a virgin and you are experienced:
1. There is a socially prevalent notion that men make love and women are made love to, so an experienced woman with an inexperienced man is going to be gritting her teeth enduring his fumbling ineptitude. Actively counter this notion. Tell him the thought of his being a virgin makes it an especially delightful encounter for you. Tell him the thought of initiating him into the joys of lovemaking is very sexy for you. Strongly convey the notion that you expect to have a great sexual experience based on his attractiveness, his enjoyment, and your skill. Make him believe that he has no responsibility for "making this work" and can just enjoy it.
2. Take the initiative physically. You kiss him, don't wait for him to kiss you. You start undressing him. You make the move in the direction of the bedroom, etc. On the other hand, don't insist on the initiative if he takes it eagerly- be appreciative, encourage it, but don't go limp and assume he'll take it from here.
3. Tell him you find at least one thing about his body particularly attractive. Stay away from "You have beautiful eyes.", but almost anything else will do: Tell him you love men with hairy chests, or tell him he has a great ass, or tell him men with strong hands particularly turn you on, or tell him you love the way he smells, or tell him running your fingers through his hair is incredibly sensual, or... something specific, not just, "You've got a great body." or "You're so sexy." After you tell him this, show him- run your fingers through his hair, enjoy grabbing the sexy ass, etc. as appropriate.
4. Tell him you find at least one thing he does particularly attractive- again, it is the specific thing that is believed and appreciated. "I love the way you kiss." or "Yes, touch me exactly like that" beats the hell out of "You're a great lover."
5. He's going to be convinced he was either too fast or too slow- the chances of simultaneous orgasm after whatever the culture has led him to believe is the "proper" amount of time are just about nil. So be prepared ahead of time to convince him that this:
A isn't a problem and B happens to just about everybody. For "too fast" tell him the first time just takes the edge off and he (not you) will have more time to enjoy the next session of lovemaking- and meanwhile he has this wonderful opportunity (of unspecified length) to learn what pleases you while he's not so distracted. Assure him that you consider "not being too distracted while learning" an advantage in lovemaking. For "too slow" tell him you enjoy prolonged lovemaking- either it gives you a chance for more than one orgasm or it gives you a chance to enjoy the wonderful sensuality of post-orgasmic sex. If it's obvious that it's too slow for him, that he's not going to make it if he keeps at it for the next two hours, let him know that he gave you a wonderful time and that everybody's nervous the first time, he'll be more relaxed and ready to enjoy it in the morning. The important thing to remember is that you need to assure him just how wonderful the experience was for you.
Tell him you want to sleep with him again; don't assume he knows it- don't make him ask, you ask.
Be sure you are both open enough to talk about the experience afterwards, even if not right away. If it was wonderful for you both and you can sense this, be sure to talk about it. If it wasn't for either of you, talk about how to improve the experience so that the next time it can only get better. If you aren't able to talk openly about what happened, then in my opinion you were not ready for sex just yet in your relationship. Communication is the key!
Remember, this experience should be an extremely special and intimate time for both partners. This list does little to accurately describe how any one experience will or should go, but rather you should use it to come up with ideas of your own or use it as a vague set of guidelines when considering what to do and how to make it go the best. Good luck, and remember, the best companion to great lovemaking is love.