One great way to help find your own voice is to develop it through the people around you. If you’re in a band, sitting in at jams, or just playing along w/ jam tracks, how you interact with these can be a great source of discovering your individuality. And it’s pretty easy, since no one else will react exactly like you to begin with. But let’s look at some ways you can get more out of these dialogues.
First, whether you’re leading or following is really more of a continuum (or should be). Think of not just following the other musicians, but also nudging them a bit (if it’s live musician’s you’re playing with. It’s much more difficult w/ recordings). Can you change the energy in some way and get the other folk to follow you? Can you take what the others are doing and use it to forge a direction that you can then bring the rest of the ensemble along on? Or take how someone else responds to your moves and incorporate that into your next decision.
Next, you have to determine how the energy is changing (whether it’s your move or someone else’s) and what you’re specifically going to do about it. For instance: maybe the drummer has brought the energy level up by playing busier. You could play busier as well, but are there other ways to follow his or her energy? What about dropping down an octave (or up an octave) to compliment their change? Or playing closer to the bridge to get a nastier tone? Or just digging in and playing louder? Or turning on your octave divider (you did remember to bring an octave divider)? Or making your rhythm more syncopated (or less syncopated)? I could continue, but start imagining what you could do. You may find that in the situation, if you keep your mind open, you’ll know what you want to do. But I’d suggest keeping a playful attitude. Try things, see what you come up with.
Also, do the same from the perspective of leading: how can you make the rest of the group bring the energy down (or up, or laterally)? What if you switch to just octaves and fifths (if you hadn’t been doing that already)? Maybe playing more legato or staccato? What about some sustained double stops? Listen to how the other members of the ensemble react to your choices.
You could also think of yourself as a bridge. What if the piano player has moved in a different direction, perhaps creating tension by playing more chromatically, or with more alterations in the chords. How can you communicate this to the drummer (assuming your drummer’s a bit tone deaf, or just doesn’t pay attention to the piano)? Maybe by playing some polyrhythms? Or dropping the downbeat? Think of how many ways you could communicate harmonic information rhythmically (then do the same to get the drummer’s thing across to the pianist).
This kind of thinking doesn’t only apply to improvised music. If you’re in a group doing original material, use the same approach when writing your parts. When the guitarist switches on the distortion for the chorus, how is your bass line going to react? Will you play driving eighth notes? Or just slam an octave on the 2 and 4? Or (hopefully) something I wouldn’t even thin of. Brainstorm all the things YOU might do in this situation. And have fun with it.