I've done a jam before, but ended up not completing the game because of poor planning, not troubleshooting or working around a major flaw with my main mechanic, and because I had another project to do at the same time. This time, I actually want to finish the jam.

What suggestions do you have for preparing for a jam? Are there skills or techniques I should be working on?

Food and exercise / breaks are already planned.

I can't speak from experience, since this is my first jam. However, I think same rules apply like with any project that is of a similar kind.

Get enough sleep and do some good planning from the moment you have your final idea written down. Make sure you break it into milestones and make sure to have playable core on Saturday. Then polish and add on top of it.

As far as preparation goes I find its always helpful to go through basic tutorials on YouTube for whatever engine you're working in. Even if you don't know what kind of game you're going to be making the extra practice can give you new tools or introduce different techniques that may be useful during the game jam! The more practice you get the faster and easier it becomes to get some of the basics out of the way so you can focus on more important aspects.

One of the most important things that will help you finish is limiting your scope. This is a 48 hour game jam. 48 hours is not a lot of time so every minute counts. Does your game really need 20 levels when you could focus on 3 really great levels that showcase the mechanic? Does it need a fancy text-box system? Make a list and focus on important features that will compliment your mechanic. 

I hope that helped & good luck! I am looking forward to playing your game!