Wait untill a big and successful concert, where the band's leader has lost hundreds of dollars and you made well over a hundred bucks, then bring your boyfriend into the band, so the band leader expends money on new color promotional materials, then it now being three or four days latter, quit with your boy friend, and ask for the contact info for all the upcoming gigs... gee that's how at least one band member left my band...
But serriously, do it openly and with some time to cover the time to replace you. Cheers Larry