I clean the engines in my car regularly. HEres what to do:

1) Make sure the engine is cool as cold water on hot metal will cause fatigue in the metal.
2) Water proof and cover your dip stick, distributor cap, and air cleaner
3) Get some simple green and a small brush. Spray the simple green sparingly. A little goes along way and try to use as little as possible. I'll explain why later.
4) scrub the dirty parts down with the brush and rinse
5) let the water flow out of the hose and don't spray high pressure streams around the compartment.
6) Make sure to RINSE THROUGLY. I mean really really rinse the simple green off good. Simple Green while excellent at cleaning dirt, grease, and grim is HIGHLY corrosive to aluminum. If you do not rinse well over time the simple green will corrode and eat through aluminum parts like your head and block if its aluminum. This is why you should use as little as possible. But don't be scared just rinse well and you'll be fine. Let the water run from the hose and flow over the parts you applied the simple green.
7) start your car and go for a 20 minute drive bringing the engine to normal operating tempature. This will evaporate all water sitting in the engine compartment.
8) when you get back use a rubber protectant like Lexol Vynlex and wipe down all rubber hose and fittings you can reach. This will protect the rubber hoses and parts from dryrotting and cracking over time.

Look at your new clean engine and be happy :-D

After this you can just wipe the compartment down every 3-6 months with a damp wrag to get the dust off.

Here is a picture of my allmost 5 year old Audi engine. You wouldn't know by looking at it