So first I clean the engine. Depending on how dirty it was, I might use Gunk or just simple green.
Then I rinse everything very well and dry it.
Next this is the only done if the painted surfaces are very very dull I use towels and strips of cloth to act as masking to shield plastic and rubber surfaces and then spray a clear high temperature resistant spray designed for under hood use on these areas (Available at auto supply stores)
Then after that drys I use the glossy Armor All on the rest of the surfaces plastic and rubber.
Finally to reduce the clinging of water and it's spotting on surfaces I give a final overall spray of the Aerospace silicone 303 to everything.
Finally I try to not follow other cars too closely in the rain and try to avoid rain altogether.
With the engine area so prepared It stays cleaner and I can give an quick easy under hood clean up when I wash the car.
I do a very similar process under the wheel wells and the entire undercarriage..
You could really eat off any surface of my former Mod Mo.