I have tried pretty much every trick in the book with vastly different results. I'll talk copper coins here.
1) electrolytes is a no no!
2) peroxide bath works ok, but not my favorite. Slow, and don't get the results I like.
3) olive oil loosens dirt and crud over time, but leaves oil in the pores which can degrade the coin over time.
4) soaking in water or detergent and using a soft brush and toothpick is good, but time consuming. Not the best as it can leave a lot of dirt and verdigris behind.
5) An ultrasonic cleaner is great, but run too long it cleans TOO well! It will clean out every pore and make the coin look very dull with a sponge like texture.
6) My number one method is a lye soak. I found that lye based oven cleaner is cheap and easy to get. After a few hours to a day or so soaking, you get a cloud of gunk in the liquid around the coin. Then I wash it under the tap until all the lye is off. If there are loose chunks of verdigris they can be picked off or run the coin in the ultrasonic cleaner maybe 10 seconds tops. Then let the coin dry completely. Maybe a day or so or put in an oven on low for a bit. Remember to try not to handle it with bare hands, the oils will damage it over time like olive oil does. If it does get handled, wipe with alcohol to remove through oils. When dry, coat with Renwax. Once the wax is dried, the coin is sealed and can be handled some.
Hope this helps!