Sleep deprivation is the key to success (kinda)!
Jokes aside, on the first two days, I try to really plan out (with pen and paper and all of that) what I'm going to do and how much stuff I can get away with without wasting too much time. This is also when I draw my assets and narrow down the game's scope.
I think it is also important to plan ahead what kind of systems your game needs and which ones need better code. Some things can be done with lazy code and work fine, but others might benefit from better coding patterns and modular approaches.
The rest is intense work and good schedualling. I was lucky to have some extra time for this jam, but had to do a crunch at the end to get sound effects done and remove remaining bugs