Elbow grease.
Actually I like Liquid Ice from Turtle wax. It is the only wax product I have found that you can use on flat black car parts that does not leave a white residue. I keep one applicator just for the engine bay which I wash after each use. I also wear a disposable surgical glove when using this stuff cause when you get it under under your fingernails its imposs......WHOA now I'm giving beauty tips.
I just keep applying Liquid Ice and wiping it off until things look clean under the hood. A little bit of Ice goes a loonnngggg way!
If you think you might some day paint your engine cover do NOT apply it to anything you are going to paint.
This will actually cross over to the Thread I started in Aftermarket Accessories / Parts under What products do you use to clean, polish, protect your car? when I get off my lazy rear end and take some photos.