

Jam sessions are places where musicians can meet other musicians, gain experience in their chosen genres and above all enjoy playing music with others in the field. However, there are some basic ground rules that all musicians should follow when attending jam sessions- particularly those that are organized by an existing group, society or band.
Musicians should always begin by making themselves known to the jam set leader or director. If the session is being run by a band, then musicians should introduce themselves to the band leader upon arrival. Usually, the leader will ask your instrument(s) and perhaps how much experience you have in the given genre. Always be honest in your response- it is easy to tell a novice and you will gain no respect by claiming experience you do not have.
Wait for the band or jam set leader to invite you onstage. It is the height of arrogance to walk uninvited into a jam session, uncase your instrument and expect immediate status as the center attraction. It is also better to leave your instrument cased, as opposed to bringing it out and displaying your technique as other musicians are playing. This is particularly directed at guitarists. It is acceptable for brass or reed players to quietly warm up their mouthpieces, so long as they are discreet.
Once on stage, it is vital that musicians listen to the other players. Jam sessions by definition are often made up of players who have not often played with each other. It is therefore important to try to fit in, and not to attempt to make yourself the center of attention. This is especially true if you are playing with musicians who are more experienced, or more skilled. You must fit in with the music produced by the other members. Do not try to overpower the other musicians. Volume and taste go hand-in-hand. I once attended a jam session where a guitarist sat through the first half of the session with his guitar out, while he engaged in impressive-appearing finger exercises. Once onstage, he turned his volume up to the point where no one else could be heard. The resulting cacophony lacked much musical integrity and was in any case much to loud for enjoyment. Play within yourself and listen to your fellow players, so as to create a musical structure that is enjoyable both to you and to your audience.
Speaking of volume, loud does not automatically equal better. The same is true of fast as opposed to slow. There have been many times where the audience might get more enjoyment from a well-performed slower piece than a fast, technical piece played with abandon. Temper both your volume and your tempos to match those of your fellow players, and the audience.
Finally, solo etiquette is very important. Wait for the leader to indicate your solos- do not simply take them, while thinking, 'Oh, it's a jam set- I can do whatever I want'. Wrong! Jam sessions are like any other musical performance- there are times to take the lead and times when it is better to play ensemble parts. When solo-ing, once again volume and taste are important. Just because it is a jam session is no excuse to go off on a massive solo that bears little or no resemblance to the mood created by the ensemble. Play tastefully. If you are the best musician in your group, it is polite to allow the less-experienced or less-talented musicians to play, so as to gain experience and confidence. I once played in a jam session led by a local trombonist. I don't recall he took a single solo, though he was a masterful player. Instead, he encouraged the other trombonists to take the solos, while he concentrated on leading the band. This was an exemplary example of a pro deferring to amateurs so as to maximize their enjoyment and allow them to gain experience.
Jam sessions are like any other musical performance and require that you follow certain basic rules of good courtesy. Don't jump right in, don't carry yourself with undue arrogance and above all, don't try to overpower the other players at the expense of the ensemble. If you follow these simple rules, jam sessions are a wonderful place to improve your technique and to make a reputation.