Great thread guys! Just wanted to say hi and throw in my $.02.
As far as gear, if I bring one bass, it's a P bass with flats. (I believe this is changing from just a few years ago, where the one bass was the J.) If you bring two basses, bring a P and a J. If you bring 3, add a good 5 string to the mix. If you bring 4, throw in a fretless.
Of course, this depends a lot on where you live, what styles of music you're doing, etc. Unless I know the person and the project I'm working on, I always bring too many basses. I'd rather bring too many and leave them all in the cases, than to say "I have the perfect bass for this song ... at home."
These days I'm doing a lot more bass sessions online, in my own studio. Since my studio is designed for a bassist (me), I always have the right gear!
If you're doing a lot of work, a good preamp or direct box is a worthwhile investment. I'm personally not a big fan of Avalons - they're ok. The Manley Vox Box sounds amazing for bass. Also Summit makes a good DI box pre. Demeters are pretty nice too. Aphex has some new gear that is worth checking out too. The industry standard is still the Neve. API too, but they are cleaner, sometimes too clean for bass. If you're just starting out doing session work, let the studio spend their money on preamps, and use whatever they have.
You should do it as long as it's fun. If you're doing it because you think it's going to "go somewhere", you're very likely doing it for the wrong reasons.
To see and hear a few of the things I've done, please visit my web site
Session Bassist Online dot com.