I had a few very awesome jam sessions of the utmost extacy so far in my guitar career (:) being 14 :)), and i can tell you you get higher than any drug will take you, and being a guitar addict, its one thing you search for.
play a song that you love, like me- My guitar instructor and myself both love the song "Franklin's Tower" by the Dead, and we jam to that almost every guitar lesson. To some, they would think "wasted lesson" but i can tell you jamming is the biggest lesson you'll ever get...
btw, dont you think there should be a "Jamming" forum at Guitartricks, like a seperate Jamming forum? Maybe we could suggest that :)