I can't believe nobody mentioned this yet... I guess it sort of goes hand in hand with "back up early and often"... but my biggest mistake (which I have made more than once) is to not do versioning. It's a real pain to have working code, and decide to add some new functionality... maybe rewrite a bit of the original code to streamline things... genercize some old code and turn it into a subroutine that can be utilized by the new code... and in the end you wind up with something that doesn't work! And here I am, with my code rewritten into non-functionality, and I've saved it right over the previous working version.

Version control. It's not just for a multi-person development environment.