In my opinion, the most important principle is the reduction of complexity by creation of good abstractions.

This includes

• understanding the problem to be solved,
• designing an appropriate solution for it and
• implementing it,
• preferably in a way that keeps the code understandable and maintainable,

but also determination of the point where to stop creating abstractions and get down to the fundamental properties of the implementation technologies (e.g. database system, programming language) to prevent creation of avoidable additional complexity.