Well, their are a few things that can cause it.
You could try a cleaning. They sell "cleaning pages" at most office supply stored. Also see
http://www.ehow.com/how_3675_clean-laser-printer.html
One possibility is a bad toner cartridge. Is it a new cart or an old one? re manufactured? If you have another cartridge, or a friend with the same model printer, you could try swapping it out.
When did the problem start? Was it tied to a particular event? Did it just start happening, or was it a gradual process? What is the page count on your printer?