Thanks for giving this article a read, using the following steps have never failed to help me and plenty others, and I hope that they do the same for you!
Let's face it, if you're unpredictable, or if you yourself don't even know where you're taking this jam, you'd be an incompetent jammer! Easy as that. Your 'jamming partner' should be able to feed off of you, and if you yourself can't even develop an improvisation to a place that YOU are familiar with, obviously your 'jamming partner' can't. So you see, the greatest jams ARE pre-meditated, but they still are imporivsed. Lost yet?
You see, as in touch as you and your 'jamming partner' may be, you and he/she still have to get acquainted as musicians. This means A LOT of trial and error. Ask yourself, 'What is it that we do that triggers our chemistry? How can we do that again?' Maybe there's a special harmonizing that you and he/she do, or some cool syncopations that are worth being noted and possibly utilized in a live jam.
Be it with a crescendo, a little reference point that tells him/her that you're about to take it loud or soft, you and your 'jamming partner' should have signals, ways of staying in touch throughout the jam. If you guys fall out of communication, then obviously the music won't mix. Sorry to seem biased, but if you're into the Red Hot Chili Peppers, you may notice that Flea and John Frusciante stand right next to one another when jamming, staring into eachother's faces, so that they both know where they are and where they're going. You and your 'jamming partner' can use facial expressions, a gradual changing in the music, or even verbal communication during a jam.
Make Rules For Yourselves!
A great way for you and your 'jamming parter' to be on the same level is to set standards for the jam. 'OK, you have to stay in 4/4 timing, I have to stay in a certain key, we need to clock our tempos so we say snycopated, you can have X amount of chromatics, but don't go overboard...' etc, etc, etc. This helps to unite the music and make it flow. It's also relieving to ALWAYS have something to refer back to when jamming.
Practice, Practice, Practice!
This refers back to number 2, but can not be stressed enough. Say you're planning on going into a jam directly after a song at a gig. Obviously, you and your 'jamming partner' will NOT have identical ideas. Maybe you're thinking, 'OK, a quiet little progression he can play while I do some slow, legato soloing with some mellow drumming behind me, and then, after about 64 measures of this at 72 bpm, we might end it with a nice little fade out...' while the other guy is ready to pull the most distorted, insanely fast progression the western hemisphere has ever seen at 300 bpm, to end after 128 measures with a sharp stop.
There are simply too many variables!
So, again, you and your 'jamming partner' should practice a few sample jams, and compromise on a general essence that you guys can get into. You don't have to be predictable to your audience, just to eachother.
Keep It Simple!
When starting out, you should know that the less musicians invloved, the easier it is to organize ideas. There is less a chance of a miscalculation when playing live, and the music won't sound as 'all over the place'-ish. Of course, you should also begin to escalate by adding more musicians when moving on to different levels of jamming. And remember, the same steps apply!
When starting out, you should also give at least one musician an assignment - just stay with X chord progression while the other maybe could solo over.
Practice With Yourself!
If you have a recorder or a means of playing back a track, you should try to experiment as much as possible with yourself. It makes you generally more competent, and gives you a deeper understanding of music in general. Recording, playing back, and playing over a track often times can lead to an original song, as well.
Know When To Quit
Im sorry, but sometimes the chemistry just isn't there. Maybe a sound can't be agreed on, or you and your 'jamming partner' just can't understand one another musically, but you should be able to accept the truth. However, if possible, you just need to identify specific issues and address them as soon as possible.
I hope that this article proves useful for all looking to jam!