Use a pressure washer on mine. It sits outside under the trees and the pressure washer really helps get rid of some of the tree gunk and the occasional bird poo. However, I never use the high pressure nozzle nor a direct spray.
Every 10 hours I wash her off, check/refill all fluids and spray WD-40 on all the 3PH moving parts. I use the "Krud Cutter" cleaner degreaser and soak down the hood, wheels, tires, floor board and a light coating on the lower and back part of the engine and transmission. Then rinse it off with medium pressure. Depending upon how much dirt is caked on usually takes 15 to 20 minutes.
I like what GuglioLS once said that the only parts on a tractor that should be clean are new ones, or, ones recently worked on /forums/images/graemlins/laugh.gif. I use to say about my old truck that a dirty truck is a happy truck, believe the same is true for tractors. But, a clean tractor sure does look nice. Plus, it's fun dirtying them back up again.