I've participated several times, so I like to share my experience and gained knowledge with new jammers.
Get started early
In the jam, you're allowed to start with any base code. So, make anything you can. I keep Unity packages with basic features that have nothing to do with the theme, like pause/restart buttons. During final round for theme selection, look them over and come up with ideas for each potential theme. See if you can think of ideas that will work with more that one.
Scope your game properly
You have three days to make a game? Try to design something you can make in only one. It'll take twice as long as you think, so you'll actually finish it in two days. The final day is for bug testing and polish. If you manage to finish everything WAY early, you can expand on the completed game. It's better than rushing or running out of time.
Be healthy
Get sleep. Eat three meals a day. Spend time with your family. Staying awake for 72 hours isn't going to make your game good, and it's definitely not good for you're well-being.
Easy games are appreciated
Jammers will play your game for a minute or two, vote, and move on. Make games that can be played in their entirety in a short time, or at least will show the player all it has to offer.
Small teams are okay
My best games were made by myself or with one other person. Too many developers just get in each others way. If you have a large team, might want to consider splitting into smaller teams.
Accept and embrace criticism
Some people get defensive, but it's really not fun or helpful. Some jammers are afraid of being critical, fearing retaliatory bad votes. Critical feedback is the most honest and most helpful. Always remember, it isn't the player that doesn't understand your game, it's that you didn't understand your player.
Make a web build
It's the easiest way for jammers to experience your game for rating/reviewing, and that's the most important. People who aren't using specific OSes, or who just don't want to download hundreds of new games that they'll play once, will be very appreciative. You can still make downloadable builds as well, but at a minimum a web build should be made when possible.
Simplified controls
It's actually a bit surprising to me how many jammers don't have gamepads for their PCs, but we all have keyboards and mice. You should make your games at least playable with just those, and as simple/intuitive as possible. Pay attention to regular conventions, having to switch from WASD to arrow keys between games really interrupts the flow of rushing through many games. It's pretty easy to have your game controllable with both.
You have to play games, too
You aren't even in the ranking unless at least 20 people vote on your game. The best way to get voters on your game is to play, vote, and comment on other people's games. I make it a point to play the games of everyone who comments on mine.