For future, it depends on the composition of the coin.
For copper the following may work -- hot peroxide, nic-a-lene (an over the counter acid to clean coins), or I just had great success with an awful indian head by rubbing it with barkeeper's friend; Sonication does not work as it will take the surface and all details off. Sometimes a long several week soak in mineral oil helps as well. I like using hot peroxide on a toothpick and scrape. There's also the old rock tumbler, but that wears detail off as well.
For nickel, Sonication gets it clean, but will leave rust color from alloyed metals, bar keeper's friend rubbing in hands for 10 minutes does awesome. Others soak nickels in hot worchestire sauce and brush occasionally (takes a little while, but if you do it too long, it'll eat the coin away).
For silver, sonication cleans w/o damage; i typically never do anything to tarnish or stain, but some people use a baking soda rub, but this ruins the coin's value if its a key date. My other trick, if its silver with tar or concrete, etc that wont come off, use invisible tape and put on coin, depress/pull off and repeat
Then make sure to put in acetone to get adhesive off.
In all cases, a bath in acetone is good to get off any surface junk (ie oils, dirt, etc.)