Sleeping with best friend ?

First of all, you need to laugh about it with him/her and act like it was no big deal and you didnt' mean to do it! Talk to him/her about it, tell them how crazy you feel and ask them thier thoughts. If you really are best friends, then you should feel comfortable talking about it. Tell him/her striaght up that it was an accident and that you don't want it to ruin your friendship. Be open and honest. It's your best friend. And use a bit of humor and laughter in it to break the ackwardness. Things will work out somehow.