Better tools, better code writing help a lot
Something that found to be really important is to use better tools. For example, if you don't have a good tool to handle documentation (and no, I don't mean simple "//" comments, I mean full "///" comments at the very least), code rot speeds up. If you don't use a *good* version control system, code rot speeds up. If you write dense, unreadable code, code rot speeds up dramatically. If your code is written with a pile of hardcoded values, code rot gets VERY quick as the underlying rationale for the selection of the values is forgotten over time, and people just slap on more and more layers of junk to override logic in new cases. And on and on and on.
For me, I've found that using the non-mainstream systems (and there are lots of good ones out there, I'm in love with Agile Platform, but I've seen a lot of very good ones out there... Django, Ruby on Rails are two examples) really address these issues well. As your code becomes more easily maintainable because it's obvious where to make changes and it is easier to make changes with metadata, not hard values or conditional statements, code rot slows down quite a bit. I've seen tons of .NET projects that had significant amounts of code rot even before the initial release, it's a lot harder to have just that bad in the more modern, less tradition-bound systems.
J.Ja
Contributr 
