I recently met with a guy who is responsible for auditioning players and filling band slots in Nashville. I asked him how he finds players and he said he has his own first-call list that he uses religiously, and when they aren't available, he get recommendations from his first-call list players - most of whom have their own short list of first-call subs.
So obviously living in a music mecca (like Nashville, USA) is very important. Then networking and establishing yourself as a reliable, go-to guy is critical so you can hopefully get on some A-lister's A-list. That probably means getting out there every day and night going to gigs and meeting other musicians - going to as many auditions as you can and really getting your name and face out there.
I also asked him what he looks for when a bass player comes to audition (besides the ability to play). His very first answer was "Pro Gear" - he went on to say that he's seen people arrive with their instrument in a garbage bag or their rattle-trap amp in a little red wagon. Those players may or may not be able to play, but the quality of their gear put them on the 'no go' list almost immediately.
He then said that general appearance was important. He said that didn't mean that you had to be a model or look like an athlete - although that certainly doesn't hurt - but you should definitely not show up in your ratty jeans and holey tee-shirt looking like you just rolled out from behind a dumpster. - This was for doing touring stuff. Obviously an audio recording doesn't care how you look.
I asked if reading was critical - he said it was important since a lot of the players he auditions had to read from charts and sheet music frequently - but he also said that while it was important, it wasn't necessarily critical.
He also said that beyond passing all the individual tests (playing, pro-gear, good hygiene) that you have make a good connection with the rest of the band. You could be the best player of the bunch, but if the band doesn't 'dig you', you won't be coming back to the next session.
Above all, once you've been given the opportunity, you absolutely MUST be ON TIME and READY TO ROLL 100% of the time! He said he's had a couple players who came very highly recommended miss a start time - even by a few minutes - and that pretty much did them in. He said one player even called and said he was right outside the door - apologized profusely for being late and was told "too bad, so sad..." his number was removed from the little black book and the next guy in line ended up getting the gig.
So - beyond the requisite playing skills you must:
1) Get into the scene, press the flesh and make friends in high places in the hopes of becoming one of their recommended subs should the opportunity present - and if you do get recommended for a gig, DO NOT BLOW IT by being late or unprepared! Not only will that get you eliminated from the gig, it will reflect poorly on the guy who recommended you which may or may not harm his gigging potential - but at bear minimum will ensure he won't be tossing any more recommendations your way.
2) Have pro gear that is always in working order
3) Be presentable, well-spoken and ready to work
4) Be punctual
5) Get along with others
Mind you, I am NOT a first-call session player. But I do try to behave as if I were one which is why I asked this guy all these questions. While I don't think I'll ever put myself into the market as a session guy, I want to at least feel like if the opportunity presented itself, I wouldn't drop the ball.