If you clean your coins improperly (which is very easy to do) you will significantly reduce their monetary value. One of the best and fastest ways to ruin a valuable and rare coin is to use a silver dip type cleaner, or rub it with something abrasive like Wright's silver polish. (For more ways to easily ruin your coins, see the Top 7 Ways to Ruin Your Coins.) Believe it or not, your old coins are virtually always worth more with the toning on them, than with the toning removed. Better yet, don't let them get toned in the first place!
Having said all that, sometimes it is appropriate to give certain coins a careful washing with mild detergent and warm water. For example, if you decide to help a youngster start their first coin collection using the advice given in How to Start a Coin Collection on $4, gently washing the coins is a good idea because coins in circulation are very unclean, carrying all kinds of germs and filth. In fact, about the only time it is correct to clean coins is to give recently circulated coins a good washing for hygenic reasons. But don't ever immerse uncirculated or proof coins in any substance to clean them, not even water!
If you have some coins that you think need cleaning, follow the easy steps in How to Safely Clean Your Coins, and if this advice doesn't solve the problem, come ask for help in our Coin Collecting forum, before you ruin them!