Comment! Comment! Comment!
I had a colleague at a place I used to work thank me one time because after I left there he had to document a program I had written. He said he literally just extracted my comments and "tweaked" it from there (since things were broken up into logical routines he had to tie those into the proper place in the main paragraph documentation). Still, he was quite happy he didn't have to try and "figure out" just what the heck I was doing in there. I know this is tooting my own horn, which I generally think is pretty uncool, but I am particularly proud that I tend to produce very easily maintainable code.
I know it takes extra time and some people don't like to do it, but I think any programmer that has to maintain something a year after you leave (or even you if you have to maintain your old code - it's hard to remember what you did a year ago sometimes) will definitely appreciate it.