One of the best ways to learn to jam is to put on your favorite CD's and just start playing. This helps you train your ear to quickly pick up chord changes, rhythm, and melody, even if you don't necessarily know the song well. Improvisation is a skill that only comes through practice, but that doesn't mean you need a full band with you every time you want to play.


Remember to learn the rhythm and backing parts of the songs as well, not just the solos. To succeed in a jam, you need to be a part of the band as well as a willing improviser.
If you play with several musicians regularly, ask them for 4-5 songs that they want you to learn, and offer them a few you like to play. The next time you meet your band will have a few more songs to jam on., If you're going to a gospel/bluegrass jam, you need to know how to play "I'll Fly Away" and a Johnny Cash song or two. If you're going to a rock or blues jam, you need to know the classic 12-bar blues format ("Stormy Monday," "Everyday I Get the Blues"), and a few Beatles or Rolling Stones songs that everyone knows. If you're a jazz musician, you should have "Summertime," "Heart and Soul," "Bye Bye Blackbird," "Round Midnight" and several other jazz classics down pat.Don't feel like you need to memorize tons of songs before you're able to jam. As you start playing, note the songs that come up frequently and make a point to learn them back to front. In many cases, jamming on a song is actually the best, and fastest, way to learn a song., You should be able to find most notes on your instrument with ease. If you know where everything is you'll be able to quickly learn from others while jamming, picking up new melodies and chords for songs you don't know so well. You need to spend plenty of time practicing on your own, not just showing up and trying to wing it. The better you know your instrument, the more you can stop thinking and start playing.
 You will never know every single chord or song that someone suggests, but having a good mental catalog of songs will help you quickly adapt to whatever song is being played. While it is by no means exhaustive, there are some common chord structures that any band could start with:


A - D - E (Frequently in 7ths for blues songs)
G - C - D
C - F - G
G - Am - D
C - Am - Dm - G, While studying theory may seem like the antithesis of good improvisers, quality musicians know that music theory is the secret weapon that helps them adapt in any jam. Knowing song, chord, and scale structures allows you to figure out songs on the fly because you can quickly predict where the song is going. Chords are not smashed together randomly -- there are certain principles and formulas that dictate what sounds good together and how each scale interacts with certain chords. If you want to be a quality jam-player, you need to do your homework.
Jam along to your favorite songs at home.
Learn the "standards" in your genre.
Know your instrument.
Learn a few common chord structures.
Learn music theory.
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