Before you spray with water:
I don't like to spray down a hot motor. If possible let it cool for 15 to 20 minutes if you have driven it any distance. If the engine is cold, then start it and let it warm up some. 10 minutes is plenty, and let the engine cool a few minutes so the exhaust manifold isn't hot.
A good idea is to spray down the dirty areas with a product like simple green. This cuts dirt and grease well and is easier on the paint/connectors and environment than typical foaming engine cleaners.
Use a soft bristle paint brush to work on stubborn spots, a brush can also get into crevasses better than your fingers in many instances.
Next if you have a hose available, gently rinse the area and see if you got most of the dirt. Touch up any spots you missed. If using a high powered car wash wand, then use only the rinse cycle and don't concentrate on the ignition parts and don't spray directly at any exposed air cleaners or air intakes. If necessary, put a plastic bag over those components. It's also a good idea to cover traditional distributors on older cars. The coil packs of newer distributor-less ignitions generally don't need to be covered.
Be sure to rinse off the fenders, especially if you have rinsed off any battery acid!
When through rinsing, take old towels and wipe down any shiny painted surfaces or those that might spot, like manifolds or chrome accessories. It's a good idea to start up the engine as soon as possible and let it warm up. A brief drive will also get rid of puddles that could start rust.
Take rags and wipe down any surfaces you can get too.
