Sorry for the slow reply. I don’t come on Tumblr very often and been busy over the last few weeks!
Back to the question.
Advice I would give would go for practically everything in music.
Contacts - the more people you know in the music industry the better. The majority of sessioning I have done has been for people who I have known for a while. When you have your name out there as a reliable and competent bassist, work will come easier.
Theory - learning music theory -although not necessarily required- is a huge advantage, especially when it comes to higher demanding work where sheet music may be given and you may be expected to use that without any rehearsing.
Gear/transportation - it is important to have suitable gear and transport. No use turning up to a 500+ venue with a 20watt practice amp.
Not being a dick - sounds somewhat condescending but the better you are to get along with the more likely people are to keep you in mind for future jobs.
Versatility - The more genres and styles of music you can play the better as it opens up different types of gigs for you.