Keeping files off the OS X desktop is a simple way to improve system performance. Why does this make a difference?
“The operating system treats each desktop icon just like a full-size Finder window—the icon takes up a chunk of memory, and the system has to track its position and size at all times. Drop enough files and folders on your desktop, and you may start to notice side effects (such as spinning beach balls) when you’re trying to do something as simple as open a new Finder window.”
The act of maintaining a clutter-free desktop also encourages you to file away useful documents for retrieval when needed. So be sure to put them in some kind of clearly named directory structure that will allow you to easily find things when you need them.