As someone who's never done a trijam before, as well as someone that's been outta game dev for a while, I'm preparing by planning out what I want to do for my game. Creating a design doc, thinking out level design, concepts, the whole nine yards!
This is my first time doing game development with other people, and for 2 of the three I'm working with, this is their first game jam! I'm also going to be the only person of the four that's going to have access to the Unity project, so I've REALLY got to set everything up to be as efficient as possible.
If you're working on a team, I'd recommend you figure out who's doing what.
If you're working by yourself, you should plan out the project you want to make, and then make a list in order of priority of what needs to be done :)
If you have any more questions, or even any advice of your own, feel free to lmk!