Well, don't spray it with a hose. I'm sure people will give you some good products to use, but the last time I cleaned my engine bay I just used a box of q-tips, a roll of paper towels, and a bottle of all-purpose cleaning spray. And some Armor All to shine up the plastic cover. It came out looking damn good too. As with most things, time and attention to detail will make the biggest difference between a good job and a half assed job.