simple green man and elbow grease. nice thing about simple green is it's biodegradable, cheap, readily available, and can cut through some serious grease/grime.

wd40 will cut through a lot of stuff, but i'd think it would attract dirt and make the problem worse. they do make sprays to cover your engine bay with for protecting it from dirt, only thing i don't like about those is they leave a film which gets on you whenever you touch it.