Commit to it
If you're going to do a game jam, make sure your schedule isn't busy. When the theme is revealed, start thinking about ideas and make sure to write them down somewhere to not forget.
Games don't have to be perfect
Games here are prototypes and don't have to be fully fleshed out. Make anything (as long as it corresponds with the theme and rules) you want and release it!
Take breaks and stay hydrated
It's easy to get lost on your project so it's a good idea to add reminders to take breaks at some point. Stand up, stretch, go outside for a walk to clear your mind, and stay hydrated.
Have fun
Games jams are all about having fun and making something you enjoy. When you release your game, you'll feel proud on what you created and that will motivate you to participate in more jams!