If a band is not the right fit for whatever reason (i.e. musicianship, material, too many gigs, not enough gigs, etc), how do you go about turning down an invitation to join? The majority of time you want to do it an a tactful way as to not burn any bridges. Of course, if you don't care then it doesn't matter.
I've only "quit" one band I've played in over the fact I just didn't want to play with them. Both of them involved band leaders who were unorganized and generally unlikeable human beings. The first one was a guy that claimed to have some really good musicians hooked up with him and just needed the final piece. After 4-5 practices it was apparent these other musicians were not involved in the project. He would make up stories such as "so and so had to move today" or "so and so's wife/GF is in the hosptial". On top of that he had different people show up each time. Some of them were actually pretty good. When I told him I was no longer interested in pursuing this project because it was not for me he called me several times and e-mailed me about 7-8 times demanding an explanation. I don't think he ever got out of the garage.
I'm not sure if you'd even consider the above a band? Other bands I've quit for a variety of reasons (better opportunity, relocated, etc) but the band leaders in these cases were cool about it.