Yeah, I would just completely forget about covers when you jam unless its a song everybody knows by heart. You will end up getting lost no matter what IMO and its no fun that way. Just start playing some random progression or riff, then have your bass player work off of that and before you know it, it will probably be some other completely different jam, and it usually sounds pretty sweet. I just have or bass player jam with the drummer really and then I just solo or play random crap in the backround (which is usually how these songs transform into something different). Works out well for us.