* Make it short first. If your idea is to have a whole set of levels, like a world in Mario 3, cut it down. Aim to make one level first. Most people won't play your jam game for more than 2 minutes
* Focus on the important tasks first. I remember back when I did my first game jam I spent a whole day making a shadow system, and by the end I had to rush the combat system which was the most important part
* Plan out your game. Write down your ideas and mechanics in a way that seem doable
* Make your game easy to finish. For example, put checkpoints and avoid giving game overs