Chances are you have some sort of metal on your altar; perhaps a brass censer or candlestick, or something made of copper. If so, you also know how quickly brass and especially copper start to tarnish or look a bit dull. Doubly so if things are getting handled a lot or sprinkled with salt water or wine.
A handy tip I picked up a few years ago for cleaning metal is to make a paste of equal parts salt and lemon juice and use it as a cleaning solution.
Please note this only works on real brass, copper, etc. I have not tested this on plated metal, and I don't advise you to try it either.
The salt acts as an abrasive and the acid in the lemon juice cuts through dirt, oil, and tarnish. The two combined with a little elbow grease leave the metal very bright and shiny. Ketchup will do the same thing for brass, but it's very messy and smells foul.
Copper tarnishes much more quickly than brass or other metals, and if your pentacle is made of copper, mine is, then you're likely to have lots of tarnishing considering how much handling it receives. Add splashes from salt water, and you've got a lot of polishing to do.
So there you have it! An easy solution to making all those metal tchotchkes look show-room new.