First of, its not really about finishing a complete game. a small working prototype of a game idea is more what most gamejams are targetted at.
I usually try to make a list of things i need to do to get my game done and then make a plan in which order i will tackle them. That helps me to be a bit more organized and get at least something done.
Joining the Chat and talking to the other jamers is also a great way to stay motivated/focused. :)