Also, document the systems you develop and comment your code. I can't stress how important this is for any project with more than 1 programmer - and even for those with just 1 programmer. It's said that more than 80% of development teams don't do this. Without documenting the system and no commenting, new and existing members of the project will constantly introduce new bugs and cause new problems.
Undocumented code can however keep you in a job! ;)