Learn how to leave a job and make the transition easy.
If you have files related to old projects, send them off to whoever might need the materials for reference. If it's your successor who will need them, list the projects they were related to in your transition document and why someone might need them. Remember you have computer files, too. Arrange those in folders by project or client and again, use the same names as you use in your transition document.
Whatever's left, toss. Unless your company has archives. Then, send your stuff there. Babson's archives includes Sir Isaac Newton's fore-parlour. The actual fore-parlour itself. In case I don't feel inferior enough compared to the undergrads who are running $30,000/month businesses from their dorm rooms in “e-tower” (that's e for entrepreneurship), I can always hang out by Newton's fireplace, and remember he invented calculus at age 19 to help him invent physics, and all I did at age 19 was fail physics.
Gather Your Junk
We've all got our junk. And my junk is ... whew! Books and staplers and pens and sticky pads. Junk that belongs to work, I leave. My junk, I bring home or give to co-workers. The stapler from the infamous Clown Incident? That goes to Heidi. It's the least I can do, since she needed almost a year of therapy to recover. Throw the rest away, sell it on Craigslist, or give it away on FreeCycle.org. You can even find a taker for your velvet Elvis poster.
Clean Your Computer
Did you ever install personal software on your computer, bookmark personal web sites, or create personal documents? Make sure to clear all that off before leaving, unless you really want people to have to dig through your experimental recipe for chicken noodle cake with cayenne sauce. That one could be a reputation buster.