I use a product called engine detailer....it gives a nice sheen to all the rubber,plastic parts ....engine must be clean first as this is not a de-greaser....as far as alt and valve covers...they are not supposed to be shiny...just clean.....

I've used the Simple Green avaition spray (it's blue) to clean them up first.  Also a wire brush attachment on a power drill.  Then try some 00 steel wool and a wire brush. Use a putty knife with the steel wool to get into the hard to reach nooks.   Lots of metal polishing products out there to use for shining up the cases.  It's tedious but rewarding. 

Steel wool flakes are electrically conductive. Get those inside an alternator, and things may get exciting. Or your alternator will be clean, be cause you'll be buying a new one when the old one shorts out internally.

You didn't specify which "car shows" you're prepping for. For show'n shine, or NCCC stuff wire brushes in drill motors :rolleyes:, polish, spray detailer, etc may be fine and desirable. For NCRS, these techniques are detrimental. They change the finish appearance, and you WILL loose originality points for "finish". 

When I prepped my 85 for NCRS, I disassembeled the alternator to bare cases. I cleaned the cases, and the stator with detergent and water. I then sprayed the cases with a mild solution of "Metal Prep" which contains phosphoric acid. I brushed it lightly with a bristle brush and rinsed it thoroughly with water. Dried and re-assembled the alternator. It look brand-new, consisted of all born-with components, and received zero deductions during 3 separate, subsequent Flight Judging's. I used the same techniques for the aluminum suspension components, although they were not removed from the car during preparation.

Good luck at your shows.

Steel wool also leaves behind lots of stuff, the fibers left behind by the steel wool get rusty when wet.  Very messy.  I have also gone the route of dis-assembling my alternator and lightly bead blasting the aluminum.  Very low pressure used with glass bead so that you don't remove any details or round off any edges.  Then re-assembled, it looks like new.  It stayed looking good for a few years, even though I never drive in the bad weather, it still began to oxidize and start to look dull again.  This time around I just thouroughly cleaned it with a wax and grease remover, then carefully masked everything and gave it a very, very light coat of Eastwood's AlumiBlast spray paint.  Just a light coating to brighten it up but not to look like it's been painted. I was very happy with the results and have gotten lots of comments on it.

My friend a NCRS judge told me to lightly buff with 0000 steel wool, use engine degreaser only in areas needed gently hose off than get CRC POWER LUBE spray entire motor wipe off excess and leave alone. The judges can tell resprays. He also said don't make nonpolished engine components perfect the NCRS judges will take the millage (10,700) in account. Don't throw up red flags. I'm doing this for both car shows and the NCRS Florida regionals in Jan. 2013. Does anybody agree or disagree?