This will be my 8th time participating. I've decided to offer advice on some things I learned the hard way. For this post I'll focus on stuff to consider before the jam even starts. Anyone can feel free to chime in with advice or questions:

Get a head start: But isn’t that against the rules? No, actually (at least not for the Compo). The rules state that you’re allowed to start with any base-code you may have. At the very least, create your profile on LDJam.com and a placeholder for your project on itch.io (or wherever you’ll share it), so you’re not rushing at the end. Basic reusable mechanics (ie a platform character controller) are fair game. I also like to prep some quality-of-life stuff like a pause menu, blank title screen, and quit button.

Avoid too many cooks: I've worked solo, with a partner, and with teams of 3-7 people. Small teams with segregated responsibilities are okay, but too many people tend to just get in each other's ways.

Start brainstorming: We don't know the theme yet, but voting has started. Copy the possible themes to a text file and start typing up ideas of games you'd make for each of them. You might find the same idea works with multiple themes, will little to no tweaking.

Scope your projects accordingly: I've had to bail on a couple jams, because I bit off more than I can chew. My new rule of thumb is to plan a game that will take 1/3 the time I have allotted. I figure I'll hit unexpected snags that will make it take twice as long, and the extra 1/3 of the time is great for bug fixes and polish.

Consider quick and easy games: There's thousands of submissions and you'll be lucky if anyone plays your game more than a minute or two, so make that as impactful as possible. They should be able to experience the entire game, so make the controls and mechanics as simple as possible.

Be healthy: Don't plan to stay up the whole time, and live off junk food. Treat it like an ideal work day. Eat healthy, get rest. Neither you or your game will benefit from burning yourself out.