Your friend is into you, but you're not into him. Handle this sticky situation right and you won't have to sacrifice the friendship.
Use The Word Friend - A Lot: Tell him how glad you are that he's your friend, and let him know what a great friendship you have. Unless he's oblivious, you'll get your message across without risking resentment from him or shattering his ego with an outright rejection.
Be His Wingman: Take an interest in his love life or offer to introduce him to your single friends. And while you shouldn't torture him with TMI about your relationship, don't be afraid to tell him about dates you go on. It reminds him that you're both on the market - separately.
Be Honest, But Sensitive: If he's not picking up on your clues, you may have to be more direct. Say something like 'I care about you, but I don't feel that between us.' And if he asks what it is about him that you aren't into, don't take the bait. He doesn't really want to know.
Expect a Cold Shoulder: Once he knows the deal, he may need to put some distance between the two of you for a while. Don't take it personally. And avoid trying to comfort him - it'd only rub salt in the wound. You can ask him if he wants to talk, but if he says no, wait for him to come to you.
Maintain Healthy Boundaries: When he resumes the friendship, remember that he is, in fact, a guy. That means no touchy-feeliness and no, 'Oh, it's no big deal if he sees me in my bra.' You'll only be igniting the spark all over again.