I've never used this for copper pennies, but boiling 2 cups of vinegar, 1 cup of water, and 2 tablespoons of baking soda with copper in it will clean it fairly decently. Using a soft bristle toothbrush it will clean a lot off. Then I've used Wrights Copper cream to really clean it up after rinsing with distilled water.
Now like I said, I never had to do this to a copper penny, but I have used this many times in the past with copper feet for my antique clocks.
Boiling peroxide I've heard works too, but I've never personally tried it. Maybe this weekend I'll give it a shot.