as songs peak, play simpler lines way low and dig in really hard, on the B or E string, depending what you've got. if you have a shredder or a great soloist then that will compliment what they play a lot better. just ride the energy waves and provide those low and loud notes for a little while.
a big key to improvised music is dynamic contrast. No matter how many cool notes and grooves you can play, everything at the same level is boring. vary between heavy loud grooves and really quiet breakdowns and everything in between.
I know I'll get flamed for this but Phish was AMAZING at their communication and peaks/valleys in their playing. Dunno if you sound anything like them, or even want to, but their skill at that facet of playing was simply incredible. worth a critical listen.
Watch what the other players do and play off them. Sometimes it's fun to do a little call and response thing. Even just mimicking the rhythm works well too. In my jammy band, I am always trying to fuse what I'm playing with what the other guys are playing. Although sometimes I gotta lay something down that's all me and let them play off it.
In summary- a good jam is comprised of a few things, in my opinion. There are lots other things involved, but these I consider critical.
1. LISTENING to the others and playing off each other
2. Dynamic Contrast as a band
3. interesting licks/grooves
4. Technical Skill
one last thing. there are NO wrong notes, only wrong facial expressions. heh heh heh
if you want to hear some of the stuff I've done, check out "Blue Light" on my Cinnamonforce page (right below). That is, I think, my jam band's best recording to date.