It depends what you want to do.  Of course, you should start to gather a team before the jam. If you want to learn a new engine before the jam, feel free. Basically, any preparation that doesn't involve making the game is fine. 

What you can't do is write down ideas for the game, start thinking about what features & such will be in the game. Natually, it would be impossible not to think of ideas, but it would be better if nothing was down on paper until the jam starts.

Thanks, and good luck!