I've left two bands, first never got past the rehearsal stage with that lineup. I knew the drummer and guitar player from a previous incarnation. This was during the time my father was fighting his losing battle with leukemia. I sat down with those two face to face and told them I was leaving and why. There were no gigs so it wasn't a big deal. After the next practice, the lead singer (the problem) convinced them to fire the drummer.
The drummer and I then struck out on our own and recruited other players. (a side note we posted an online ad, which I wrote, and in the copy said we didn't want those with delusions of grandeur or who took themselves too seriously. No mention of the other band. Well the lead singer sent me an incredibly nasty email, I guess he recognized himself in that comment. The really low blow was when in that message he said how he was sick of all the stuff with my dad. Dad was in the hospital when I got that email, and died about a week later.)
Anyway... that next band got decent, but I have limited availability, it's a hobby for me, and I make sure everyone knows that going in. they wanted to play out more, started doing so under another name, with out me, it started getting too stressful on me so after a rehearsal I talked with the guys, and stepped aside (there was another sax player who also played with them at times). I played several more gigs with them, and that was it.
Interestingly tha band split and one half formed anotehr group. that sax player moved to Fla, and they called me to fill in. I still get called, but rarely, they have gotten used to working without a sax player, but when they need one they call me.
Long, convoluted story, but I guess the point is, don't burn bridges. ;-)