Sleep, don't over-scope your project and try to do something new which you've never / rarely done before if it doesn't end up hurting your game's quality. Also better to have at least 2 roles if you're in a team to prevent having only one kind of thing to do (might sound more efficient but the mind is pretty bad at concentrating on the same thing constantly, we need some diversity !).
Also don't forget to eat and drink so that you don't just run out of energy after a day or two when your body is done energizing from your last pre-jam day's meals. When concentrating the brain can switch to a mode where it sort of "forgets" the sensations of hunger and thirst but it doesn't mean you don't need the energy anymore (the brain is the biggest consumer of nutrients in the body !)