Ok, some people may think this is just a copycat of the "How to play effectively with a band" column, but these are my 2 cents on how to make a good jam session with a bunch of your friends and actually come out thinking "Damn, that was fun!"
First of all, you wanna get alot of different kind of players to jam with, but who can all get along and are willing to learn new things. I've been playing for a year plus now, and i've jammed with alot of different people. The most important thing you have to do is be open to other people's suggestions and ideas. I've done alot of jams where everyone just wanted to show off what they can do, and i've literaly had about 3 different songs being played by 3 different guitarists at once, and believe me, its no fun. Everyone is too busy trying to show off and show each other up that nobody is actually listening to anyone but themselves.
Next, you want to make sure you have a good bass player. I haven't really played with that many bass players unfortunately because not many people play bass around here, but i have played with 1 really amazing bassist, who knew his scales and chords and as soon as he heard a song was able to play along without having the faintest idea what we were playing. That is really important, i think too many people underestimate how valuable a good bassline is in a song.
Now you need a good drummer. You don't necissarily need John Otto there but you need someone who can play along with the tempo of the guitarists and get the feel of the song quickly. You need someone who knows what they're doing, knows how to make a few nice fills when they're needed, and if they want who can pull out a solo without killing the entire song.
Finally, you need organization. I know this sounds weird but hear me out. Once you have all the elements together, what will make or break the jam is whether or not you are all on the same page. If you have people constantly trying to change what everyone is doing then nothing will ever sound right.
Now i know what you are all wondering, what is the point of all this? Well my point is, if you can find a group of people who you can actually jam with and make some songs that you know sound good, then my friend you may have just found yourself a band. I can't tell you how many people have seen me play guitar, invited me to practice with their band to see if i would want to join, and then completely turn me off because i cant play well with them.
I've jammed with Metal Rockers, Hip-Hop enthusiasts, Classic Rockers, Blues Lovers, and some people who don't even fit into any category that I can think of. But the best jams have always come with a little bit of everything. In the end all that matters is what you can make with your friends. All that matters is whether you make music, or just noise.