Whether it is by a camp fire, in someone’s living room, or on-stage, jamming is one of my favorite parts of playing music. While there are many things un-established, having a few guidelines will help you have a great jam session.
1. Have a few songs you know very well
Sometimes when people get together to jam, there is a feeling of, "Oh now, what should we play?" If you already have a few songs up your sleeve, you can suggest those as a place to start. If possible, it is great to tell everyone ahead of time what these songs are that way all parties involved will know what to have ready.
2. Know the 12-bar blues
This is a pretty universal form of music. A key just needs to be established and then you have hours of jamming fun ahead.
3. Be Flexible
If you learned a song one way, but a member of the jam session wants to take it another way, try theirs out. For instance, if another member is soloing over an 8 measure progression, and they seem to want to continue soloing, keep on playing the chords and let them play.
4. Use the most important tools you have - your ears!
This might be the most important part. If you hear the other members are playing suddenly softer, bring your volume down and match what they are doing. This is what makes a jam session great, when all members are listening and following each other. Things are being said without using words and can make the group feel like a single unit rather than a bunch of different players.