DIY: Engine Bay Washing - YouTube

How to wash dirt, grease, or grim away from your engine bay. Make sure your engine is cool when you do this procedure. Spray the solution onto the engine and around the engine bay, let it soak for approximately 5 minutes, then continue to rinse off with hose or pressure washer. Let engine dry for a few hours to prevent the vehicle from running rough.

The soap I used in this video is called Spray Nine, but I'm sure there is other cleaners available which will do the same job.

Areas to take caution around when washing:
-alternator
-coils/distributor
-spark plugs
-spark plug wires
-fuse panel
-oil filling places (engine oil, transmission oil, power steering)
-brake fluid reservoir
-intake
-air filter

To protect any areas you are worried about water entering, you can cover it up with a plastic bag.