I've developed a fairly functional style of programming, despite my primary languages being C++ and Python. I found that if I pass all of the context to a function (or method) that that function needs to do its job, and return the meaningful data that I'm looking for, that my code has become much more robust.

Implicit state is the enemy and in my experience is the #1 source of bugs. This state can be global variables or member variables, but if results are dependent on something that's not passed to the function you're asking for trouble. Clearly it is not feasible to eliminate state, but minimizing it has huge positive effects on program reliability.

I also like to tell my coworkers that every branch (if, for, while, ?:) is a likely bug. I can't say what the manifestation of the bug will be, but the less conditional behavior your code has, the more likely it is to be bug free simply due to the fact that the code coverage during execution will be more consistent.

Go figure, all of these things also have positive effects on performance too. Win!