You could just put a 24 hour counter. And make a game by an idea you just got from the internet. I did that many times, it makes you like 50% more confident, believe me :D
What I've done in the past (and a little bit for this jam, even) is pick a genre and style I want to build a game in and figure out the mechanics of it. You don't have to make your actual jam game to learn the tools you'll need to build it; and you can probably shape the genre/style you want to make to go well with the theme once it's announced.
For example: I really want to build a branching narrative in pixel art, with a dialogue system and puzzles to complete. Kind of like a Monkey Island. So, I can find the plugins I will use, work on a graphical style (and even some potential characters), and figure out how I'm going to achieve the menu and interaction systems. I'm hoping to do more of this prep work before the jam starts...
