I spray mine down on occasion with a garden hose and avoid spraying the alternator. If I want to really make it look good I also use a degreaser (simple green or something similar) again avoiding the alternator. I spray that off then warm it up to get to dry a little then use WD-40 liberally but again avoiding the alternator (degreaser and WD-40 will kill the alternator) you can place plastic or a towel over the alternator to be cautious. I would not spray the belts and pulleys with degreaser or WD-40 (dries them out)
Then using a old rag wipe off the extra WD-40 and wipe the whole engine bay down with the now wet with WD-40 rag. Dust like to stick to WD-40 so I try and wipe it off but it leaves a nice clean shine. Then run the motor to get it hot... You will see the WD-40 steam off and may get on the paint and wind shield a bit (won't hurt either but a car wash after will take care of that and it should look great. Engine bag detailing just takes time and attention to detail.
Hope that helps.