Dependent on how dusty it is, I'd vac the bay first getting rid of the worst areas and use a small brush to agitate any dirt that has got into the the cracks and crannies. Then I usually just spray Autoglym vinyl and rubber care liberally over the bay and just wipe off.
Haven't got a recent pic of my Z4 engine bay to hand but here is one from sometime last year (about two weeks of driving after the clean) and one of a Yaris i did about three weeks ago (I'm not a fan of the 'shiny' look).
Yes I did, it's an Ultra Racing brace I bought off ebay sometime last year. The black OEM braces are there to do the same thing but I figured an extra one can't hurt and triangles are the strongest shape so...
In all seriousness, I did not notice a difference in driving on the road but it does look cool and my dash creaking ceased to be (just some minor roof creaking left to sort out now...)