My engine bay isn't quite as tidy as Ron's or Richard's. However, the rocker cover on my 4-pot isn't too shabby. It had been polished when I got the car. However, as it was starting to dull somewhat, I gave it a polish with Autoglym Metal Polish. This restored the shine and it has only need the occassional buff up with a soft rag to restore it over the last couple of years. So, if your going for bare polished metal look, Autoglym seems to do the trick.
I have polished up some metal parts in the past - I would remove the cover and use a polishing mop fitted to a bench grinder. Making sure the mop and polishing paste/compound is suitabe for aluminium alloy. You would probably then need a Dremmel type tool with polishing mops to get into all the nooks and crannies on the cover.