Just as you would on a road bike, you'll want to use the Motorcycle Safety Foundation's T-CLOCS checklist outlined here, which ensures that your motorcycle is ready for action.
But hitting the dirt can also involve dropping tire pressure (sometimes to around 20 lbs or so), in order to help the rubber become more flexible with the terrain. It's also a good idea to ditch saddlebags or accessories that could weigh you down or shake loose due to vibrations. Finally, you should also consider tucking or removing turn signals, windscreens, and mirrors, as they will easily get damaged if and when you take a spill.
