Two words: Take responsibility.
If you want a relationship to work, one which you blew out of the water in the first place, then you need to stand up straight and apologize. People are often more forgiving than they look.
Be sure to follow your apology with a commitment, and then be sure to stick close with your commitment to help build trust with the person who you let drop.
I have had the unfortunate occasion to have to deal with this a select few times, and this strategy has worked wonderfully for me :).
What have you done in this situation? Fire us a comment in the discussion below :).
Keeping your success in focus,