1. Monotonicity (and immutability)
2. Pre- and post-conditions (assumed to be true just before the runs, post-conditions are things that are assumed to be true just after the function returns)
3. Invariants: things that should be true before, during and after that code runs.
4. Isolation: every change has a "blast radius" - a change to one part of the code may necessitate a change to another part to ensure the consistency/correctness of the whole system.
5. Induction: a proof technique for handling recursive structures

As second recommendation: "you should try to write your code in a form that's easy to write little proofs about"