Here's a good exercise I think will help you:
Okay - time for the jam segment of a song in E, let's say....
So, you have they keyboardist doing some nice runs, the rhythm guitarist is doing some nice chordal stuff (maybe alternating between E, E7, E7sus4, etc.) and the lead player is dancing around the scale, as is the bassist.
First things first - it's time to listen. Each person has to determine which band member they wanna focus in on (it doesn't have to be seperate, i.e., the keyman takes the lead player, the lead player takes the bass, the bass takes the drums, etc...) - in other words, everyone could focus in on the rhythm guitarist, while the rhythm guitarist decides he wants to focus on the drums....that part doesn't really matter. When the person you are focused in on hits a riff - let's say a simple bend from the 2,up to the 4 and back to the 2 (typical guitar bend in a major key), whomever is focused in on that player should either mimic it, or play something very complimentary to it (maybe a bend from the 6 up to the octave and back to the 6?)...if you're the drummer - play something off of it. Throw riffs around, basically. The REAL fun is when you all are listening to different people (i.e., the bassist and the rhythm guitarist are focusing on the keyboardist and the keyboardist is focusing on the bassist, the lead player is focused on the rhythm guitarist and the drummer is focused on the lead player, or something like that). Then you get these intertwining "inner jams" going on. The beauty is, you can change who you're listening to at any given moment. It will work itself out, as you may notice that you, being the rhythm guitarist for example, AND the bassist are both listening to the lead player and he plays something and you and the bassist both play the same type of riff - both of you may switch your respective focus to a different player or one may stay or you may find that both staying put sounds kinda cool.
You gotta practice some interesting chops, of course. I've REALLY been working on this at our rehearsals, trying to not just run up & down a scale. Trying to find a cool melody or jumping all over a scale and adding slides, walkups or bends in there to make it interesting. Finding cool melody lines is key.
The backbone, IMO, to a GOOD jam band is having a good rhythm player who doesn't just play chords and a good bass player. The rhythm player can be keys, guitar, whatever.
Just a thought.
"Once in awhile, you can get shown the light in the strangest of places, if you look at it right." - R. Hunter
"If we had any nerve at all, if we had any real balls as a society, or whatever you need, whatever quality you need, real character, we would make an effort to really address the wrongs in this society, righteously." - Jerry Garcia