Since quite a few people having posting tips and advice I thought I'd share my main advice from over 10 years of game jamming, and it's fairly simple:

Make a really small game, and finish it!

I know I'm not the only person peddling this classic which should prove how important it is. I would say this especially true for new jammers, but trurthfully as a more experienced jammer that's often led to ambition and hubris that has left me without a finished game at the end of a weekend.

If you target a game you can make in half a day, then I think realistically you're going to find it's going to take a lot longer. Then if you do finish in a day you'll have a whole extra of day to polish and make it really special.

Finding a way to get yourself to the finish line is much more important than the "learning experience" you get from another failed project. I've already got the learnings from countless unfinished good intentions. Every time I finish a small project these days there's a lot more satisfaction sharing that with everyone than there would be from a bigger game I couldn't finish.

Ideally by the end of day 1 you're playing your whole game through in some rough form. So if you find yourself at the end of day 1 with something that's a long way to go still, I would strongly encourage you to start cutting like crazy. Get the scope down as much as you can.

Now that's all said, as a disclaimer most advice is autobiographical. I'm preaching to myself and my own past mistakes more than your unique circumstances. If this doesn't fit you, do what you want, as long as you have fun then the whole weekend should be worthwhile no matter how it turns out.