Above all, make sure you carry loads of stuff home with you at night, thus generating the false impression that you work longer hours than you do. Use computers to look busy. You can send and receive personal e-mail, calculate your finances and generally have a blast without doing anything remotely related to work. These aren't exactly the societal benefits that the proponents of the computer revolution would like to talk about, but they're not bad either. When you get caught by your boss, and you will get caught, your best defense is to claim you're teaching yourself to use new software, thus saving valuable training dollars.
Top management can get away with a clean desk.
For the rest of us, it looks like you're not working hard enough. Build huge piles of documents around your workspace. Pile them high and wide. Never answer your phone if you have voice mail. That's no way to live.