I did mine and used plastic bags over the alternator, fuse box, and battery with no issues. Do not power wash, just a nice light rinse.
I take mine to the quarter car wash and spray down everything with the pressure wash, then rinse the soap off. I've never had any issues (and I've been doing that for years to cars - though I did wrap the open air filter on my Mustang to keep water out). I've either been lucky or the electrical connectors are well sealed. Just make sure the spark plug boots are down tight against the valve covers and try not to spray water directly into the alternator and you should be fine. I generally wash mine out about every month or so.
I used brake cleaner to clean up all the dust and dirt left on by the previous owner. Anyways.... Once a blue I will go ahead and spray simple green everywhere. Under the hood, engine the whole engine bay and rinse it fresh water from a garden hose. NO HIGH PRESSURE. Then quickly I would grab leaf blower and blow around battery, fuse box, alternator, spark plugs ect... Then I wipe everything down and use meguiars natural shine on all plastics and rubber. Clean all that of nicely. Then I use a wax on the hood, and sides of the engine bay and back of the engine bay and wipe everything thoroughly. I don't care , I keep my engine / engine bay as clean as possible. You could eat of it. !! My friends always make fun of me !!! But usually after a crash wash. I simply spray on plastic and ruber clean that then wax the metal. I try and avoid using water at all on engine. But once in a blue.
i took it for a steam pressure wash. came out squeaky clean. had no issue although nothing was covered. paid about $65 locally.
Real simple [MENTION=106327]GVL[/MENTION] has the right idea. Cover the alternator, fuse box, and battery. I give mine a light rinse then spray it was Simple Green (can find it at any retailer like Wal Mart). I then go in with a small brush I bought specifically for the engine and scrub all the areas. Light rinse all the soap and grime away. Towel dry or even better air dry with compressed air. Then start her up to make sure its all in working order. I do mine once a month, actually just did it Thursday after it was covered in mud and dust from off roading.
Check this video out for a visual.
https://www.youtube.com/watch?v=LESRJrB1L5I