Oil, etc out of the passages:
What ever solvent you have on hand and lots of compressed air or feed flexible wire through and pull a rag threw the passages.
Serious oil/grease/road grime build up on the out side:
A. Submerge it in kerosine and let it sit for a few days.
B. Gunk engine bright and a pressure washer or the local car wash
C. Starit Simple Green / Castrol Super Clean, brushes.
[I faked a hot tank once with a 35 gallon drum and my forge. Filled it with enough water to cover the stuff, poured in a gallon of simple green and boiled the hell out of it. Took a couple hours, but it worked. Don't thing a gas grill or similar would put out enough BTU's to do this though.]
Oxide and corrosion of the exterior:
Aluminum wheel cleaner.... the stuff that really stinks.
While you have the intake off, do the dremel to gasket thing on the runners. Pretty simple and strait forward if you have a dremel and the stylist and a bunch of sanding drums. It does make a difference.