Session work requires a lot of practice and skill and not something you can just walk into, all my work was mostly word of mouth. You need to be able to read music and play off the cuff and out of the box.
Don't take comments on your chosen Bass line to heart and for the most part keep it simple and do what is required.
I got into it by basically being invited to do a session after I was seen playing live in The Red Lion in Brentford and it went from there. (it's now a McDonalds - But that's not my fault ... Honest)
Play live lots and with many different musicians ask friends put up notices in recording studios and rehearsal rooms and get your name about..
Don't believe its all about who you know, being successful is a LOT about what you know and today i would guess it's not so easy to make a living at by doing sessions alone either!!