Simple Green for the first major cleaning if it's really bad. The real trick is maintaining it underhood. Everytime that I wash the outside, I get underhood also. I just use some dish soap with hot water in a pump-up garden sprayer and hit everything underhood. I wrap open element air filters in a plastic shopping bag, and am carefull around obvious electrical components. I let it soak for a couple of minutes, agitate especially grungy areas, then hose off with a garden hose. Lastly, I use the blower attachment for my ShopVac to blast the majority of the water out, then final wipe-down with a synthetic chamois.
This isnt' a great pic, but this is an underhood pic of my 9 year old, daily-driven Ranger.
And this is an award that I got at a car show this past October that I'm pretty proud of--Best Under-hood Detailing. 200+ cars at this show, most were truly show cars, very few daily drivers.