[StevenB]

Teach them version control. It doesn't matter what tool you use, but getting into the habit of documenting your work is essential.

For most teams, all of the software fits inside one person's head, and they do most of the work. They know what things they've changed, and how their code works. Because of the intensity of build season, they can even keep track of things without putting in any comments. I know, because I've been that person before.

If you're fortunate enough to actually have several people working on the software, keeping track of what you've done and what has changed is essential, even if all of your code is in one place. You don't ever want to be stuck in a situation where "Johnny changed the code, and I don't know what he did, but now it doesn't work...".