How can you tell your best friend you're bi?

You should know your best friend well enough to judge what way would be best. Maybe coming right out with it in a serious way would be good, or maybe you want to first claim you're bi in a joke and see how he/she reacts. Maybe it would be best to ask and find out what your best friend thinks about bisexuality first.

In the end it's your best friend though, and if you being bi is a problem to him/her, then obviously it wasn't really a best friend.