Good topic! I have a few:
- Test your game and build testing into your development plans. In a month long jam, testing should take up at least a week. If you just finished developing your game on the last day and are now rushing testing, you will have a bad time. Also test your deployment!!!
- Quality over quantity! An hour long game with a few lulls is much less fun than a 15 minute game of concentrated joy. If you can create a decent gameplay loop and storyline in a shorter time, spend more time honing what you have, and less time adding more stuff!
- Actually try hard! Feels like it should go without saying, but it is an important tip. When time is limited, working harder in the allotted time and not letting weeks pass without devving makes a huge difference in the quality of a project. When someone invests a lot of time polishing their product, it really shows. A little bit of time each day over a month really adds up.
- Like Gensun said, you should make a minimally viable product quickly! Then go back and add things, polish things, artwork etc.