I've always tried to keep things clean under the hood but since I "show" my Volt at car meets in order to educate people about EV and PEHV, I like to keep the powerplant bay extra clean and presentable. Fortunately this is a LOT easier with the Volt than it ever was with my Jeep
I usually go over everything with a gentle spray (no high pressure streams) from a dial head nozzle set on "Shower" just to rinse off grit and pollen and whatever while using a sponge glove to wipe things down.
After that I dry things off with an electric leaf blower.
I've been experimenting with a lot of different things to "dress" the plastics and stuff under there, right now I'm trying out Meguiar's "Endurance" tire gel applied with a foamy sponge. I let it set for a while then wipe down with a paper towel and it seems to look good for a LONG time, including for several rinses after application.