Depending on your experience and comfortability; you could use stuff like simple green to mist the engine bay and use warm or hot water in a pressure washer (being uber-careful to avoid electrical connections, modules, and housings) to wash and rinse the buildup of dirt away. You'd also want to keep a good distance with the wand, too close and the force of the water could damage what it touches or easily force water past waterproofing measures at the aforementioned electrical connectors.
There seems to be two camps to having the engine on while doing this: more often than not folks perform the job with the engine on so any water that may end up where it shouldn't be gets heated quickly and turns to steam before it can seep in. Two ideals on water temperature as well... I go with hot/warm water as the temperature differential between an engine at operating temperature and spigot water could affect the metals (a la Mr. Wizard's can demo). Better safe than sorry.
If you don't feel right about going that route, bust out the rags, cotton swabs, old chopsticks/bamboo sticks; 'cause your gonna be getting in there! It's much more time consuming, but 100% better for a shaky ego. Avoid armor all or any kind of oil-based dress up sprays. They break down your underhood rubber parts and they'll wear much faster than they would normally. Tire shine is also a no-no. If you clean properly, rubber should look fine without dressups... if you just gotta use something; wipe it off before you call the job done! Those products when sprayed and left, are kind of thick and will attract dirt/dust and 'undo' in rapid fashion, all your work. If you remove components to clean around them, do so a few items at a time so you don't lose your place and stumble on the locations when reinstalling them, too. Hope that helps!
I know... tlrd...
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