The KISS principle is invaluable. Keep it Simple, Stupid. Words to live by for everything from programming and engineering to lying and furniture. Kind of a corollary of Occam's Razor.

The 80/20 rule. You can achieve 80% of the results for 20% of the work on any given project and vice versa. People who've ever written code understand this at a very visceral level.

When you are writing any kind of code the less privilege said code has to have to operate the better. 90% percent of hacks happen because developers didn't keep this in mind.

Mom has a fish and guests rule. Both stink after 3 days. This is a lot less harsh than it sounds... and, as with all rules, there are exceptions.