I've read up on this quite a bit and it just depends on what you have and how clean you want it.
I hunt the Outer Banks of NC currently and most coins are tarninshed. The coins I pulled from Florida were encrusted. And, the coins pulled from dirt in Maryland, Indiana are pretty dirty and tarnished.
What I've found that is working for me is a rock tumbler. I first used the brass cleaning media that came with the unit, and it worked well, until the rouge started sticking to the coins. Then being cheap I got some corn cobs and cut them up. They work great, put the process is slow. The pennies seem to really darken the media (corn cob), which I separate out and wash to reuse. The same happens when I clean the nickles, dimes and quarters, but not as bad.
I finally broke down and bought some brass shell casing media (Green Corn Cob and Pecan Shell w/rouge) from Gander Mtn sporting goods. I have yet to try it, but if it cleans brass it should clean coins in my opinion.
Most sites I've read suggest only cleaning with water and soft tooth brush.
And suggest you don't want to clean anything that will loose it's value from being cleaned. OLD and RARE coins and the like!
Hope this helps! Good LUCK!