Code reuse

Reuse as much of the same code as possible. This does not mean ``copy and past,'' this means the same copy of code, from the same lines of a particular file. (This feature is available in every programming language that I know.) If possible, even use the same lines/files accross different projects.

This decreases the probability that you riddle your code with bugs. It also has the advantage that finding and fixing a bug in a common piece of code automatically fixes all projects which use the code.