The engine bay stays clean because it doesn't have any leaks, but on occasion I bomb the engine bay with "303 Aerospace Protectant" to protect the hoses and make it look nice. (Think Armor All, but 303 protects rather than destroys.)

I hate how busy my engine bay is; it has to be that way so I can pass California smog checks. If it weren't for the visual test I could really clean things up by eliminating the Toyota airflow meter and going with a map sensor (MAFT-Pro), and going with a forward-facing intake like the twin turbo Supras have. (Refer to the pics below.) The car would run even cleaner with these modifications and servicability is much easier. Don't get me wrong, smog laws are extremely important to have--my opinion is they should drop the visual test and rely solely on the sniffer test. If it passes the sniffer, you should pass smog.