Most people have hit on the big ones, but I think the most underrated answer is: Comments. Seriously, comment the hell out of that code where anybody coming in

Most people have hit on the big ones, but I think the most underrated answer is: Comments. Seriously, comment the hell out of that code where anybody coming in at any time can understand what's happening. Make it so that a technical director can read only the comments and understand the entire system and it's dependencies.