before even starting to jam, its important to get along well with your bandmates. best if you all have the same music taste and if possible, skill level(not that important as the inequality actually pushes the weaker ones to improve)

keeping a band together is not very easy and many bands breakup because of this. i'm having quite some problems with my band right now too.

but for jamming, always prepare early so you don't waste time and money in the studio. stuff to do:

1) decide what song(s) you are gonna play and make sure everyone is comfotable with it
2) practice the song(s)
3) practice the song(s) somemore till you get it right
4) tune your guitar if you're bringing it to the studio
5) decide on your effects and settings if you're bringing
6) practice the song(s)
7) keep a good relationship with your mates

to sum it up, just prepare before you go, don't waste time practicing in the jam room

jamming too seriously without having fun is like playing games too seriously thinking about winning and foregoing the point of playing games: fun.

but i must admit, not being able to play a song(or a game) can be pretty frustrating, so be sure to choose a song that you know you and your bandmates can play or else your band will not go anywhere from there.

have fun !