I'm yet to do mine, but when I do I will be using Autoglym Engine and Machine degreaser agitated over the engine bay with a brush. However before you even start this make sure filter opening, sparks, battery terminals are covered up with a bag/cling film as you will need to carefully hose off the bay after.
So once all the crud is shifted, carefully hose off with a gentle stream of water - no pressure washers! Then give the whole bay a spray with Autoglym vinyl and rubber care and hey presto it should look pretty bling once dry!
If you want a more in depth explanation of how to do your engine bay or any other bit of your car, try
www.detailingworld.co.uk - there are several cliosporters on the forums there, and its a well run, informative and friendly site.