Get a good tone.
Play solid, strong, supportive, bass lines.
Lock in with the drummer.
Don't worry about "solo's" or over-playing.
Make everyone else sound good.
And they will love it.
Bingo!
Also don't be afraid to drive stylistic direction. You will be amazed at how significantly you can change the overall feel of a jam by your choice of groove. You can get 'tight and technical' and turn the thing into a Yes/Rush-ish thing or get really laid-back and loose and bring out the Marley of it all - all within the same jam.
Also - if the opportunity presents itself and you want to solo, don't be afraid to take a little time and let your solo unfold. I don't know about you, but I am no Victor Wooten. At first I used to shy away from even trying just because I couldn't "shred". Since then I've learned that being melodic, lyrical and creating phrases that are logical and relate to whatever melody and style have been established goes a LONG way to creating a really nice solo. So it doesn't have to be "shredding" to be impressive. It can be very simple and still be really great.
Oh yeah - (ironically, since I am now rambling!) but LESS IS MORE!!! Don't be afraid to even pull out entirely every so often if you feel like the groove has stagnated. Sometimes holding a single whole note after you've been grooving on a syncopated line for a lot of bars can open up the feel and prompt the rest of the group to alter their tact too. This usually brings on a really welcome change and new ideas bloom in it's wake.
But as Mojo-Man said, lock with the drummer and provide a rock-solid foundation for everyone to jam over. You will be loved and respected forever!
