A problem in group jams is that we get stuck. This is a huge disadvantage to starting the jam with a strong pattern/riff that all the group lock into. A few brief notes on a topic that I'll doubtless return to:
Firstly it should be said that many good pieces of music do just stick around one thing. It may be great and need nowt else! But what if this isn't what you want to do? My experience tells me that musicians often want the music to move, to change but just don't know how to make this happen.
One way is just 'kill' the old pattern - make eye contact with the rest of the group or give a visual signal to stop it, make an ending and stop playing. This is awkward and wouldn't I suggest lead to best results but it can work. Be careful that when starting playing again you may just head back to what you just played - particularly in pacing and rhythmic emphasis.
A far more musical and I would suggest effective way are the gradual evolutions that can be heard in music such as Can. These sort of movements need the co-operation and skills of all members of the group. Everyone needs to be confident enough to (gradually) start playing something else. I would particularly emphasise the bass and drums here. If only the guitar or keyboard changes it can just sound like a solo over the existing rhythmic pattern and tonal phrase.
A simple way I have found is to begin to emphasise different notes in the existing pattern then to use these to become a new phrase. I try to change the emphasis or to build a new rhythm otherwise it can just stay in that same rut. Similarly I put a different chord into the existing sequence and then begin to play that chord a lot and to find a phrase that works off that chord. Over time you LEARN TO RECOGNISE A GOOD THING and to pursue it.
I play off the drums a lot - there are usually lots of inner rhythms within what the drums suggest. I try to bring these out. Hopefully the drummer picks up on the new ideas that I'm doing. Maybe he or she actually stops playing and then addresses the new phrase - confirming it and giving it as a new place to work.
In an ideal world I feel the music is in a constant state of change. The observations and ideas spelled out so briefly here are happening all of the time. This leads to the music being able to move, seemingly unnoticed from one place to another. Much more to be said on this when time allows.