What to do when you like your best friend and he has a girlfriend?

You have four basic options, listed in recommended order (i.e. do the first one first):

1. Talk to him. Let him know how you feel.
2. Ignore your feelings and hope that they split up, then make your move.
3. Try to forget him, and if possible find someone else to like.
4. Sabotage his relationship; this however, would probably not create any good feelings between the two of you.