I'm in love with my ex-boyfriends best friend

Defaintly wait. He might be upset about you braking up with his friend, or maybe he wouldnt want to betray his friend like that. Who knows what guys think?

The smartest thing to do would be starting a friendship with this guy! Dont spill every detail of your life to him though...because then you wont seem interesting any more. Be mysterious. Make him want to know more! Dont make it seem like your in this for a relationship...let him start the relationship when he's ready. If he doesn't seem to like you, then dont waste your time with this guy. Find someone new.

Write this down, doll. Heres how to find out if he likes you...

Keep an eye on how many times you catch her/him staring at you.
Take notice of whether s/he smiles at you a lot.
Look for whether s/he focuses on you in a crowd.
Observe if s/he has suddenly taken more interest in your friends.
Pay attention to any significant changes in her/his behavior toward you recently.
Start showing her/him a bit more interest and see how they respond.
Have a friend talk to her/him or one of her/his friends about you.
Ask, '[name], are you interested in me?'

Hopefully you will not have to use steps 7 or 8, but if you're having difficulty reading her/him, these steps are your last resort!
Remember, this is not an exact science, and it may take a few mistakes to perfect your technique

But, remember...dont start the relationship if a relationship is to be ...let him do it.

But listen to this...i got this exerpt off of about.com : Ask the Love Lady - My best friend stole my girl!
Featured Question: My so-called best friend just started dating my ex. I broke up with her a few months ago but my buddy knows that I still have some feelings for her and that I was thinking of getting back with her. Before I could he snuck in and started dating her. Would I be wrong to try to get her back even though they are a couple now?