Write the rhythm part of the song first. Aim for something that has a catchy hook or riff; this can be a repeated guitar phrase or a bass line that stands out. Because you're writing a jam song, you want the main rhythm of the song to be something that isn't physically challenging for musicians, as many jam songs are played for an extended period. Ted Nugent's "Stranglehold" is a good example of a simple song that can be played for a long time without wearing on the musicians. The bass line carries that song's rhythm while Ted plays guitar licks over the top.
Write a solo part for each instrument in your band. A jam song should give all players a chance to shine musically, although the guitar is typically the forefront instrument, especially in a rock band setting. Keep the tempo of the song at an easy pace--between 80 and 120 beats per minute. You write jam songs so a band could play 15 or 20 minutes and still be enjoying it.
Include lyrics if you have a singer. If you plan to play the jam song live, your jam song might be the place for the singer to introduce the band to the crowd. You could also write a little section in for the singer to interact with the audience and get them pumped up.