Using sandpaper is a bad idea, since it is likely to remove any protective plating from the contacts along with the corrosion. Similarly, it is a bad idea to use acid or alkali chemical cleaners, even mild ones. An ordinary pencil eraser actually makes a pretty good scrubber for most contacts that you can access. You can buy liquid cleaner formulations for electronic contacts that work pretty well and also contain a lubricant/protectant; they are available from many wholesale electronic parts distributors.
Applying chemical contact cleaner alone is probably the safest treatment for contacts that aren't visibly corroded or oxidized. Most contacts are designed to achieve a certain degree of self cleaning due to the "wiping" action when they are disconnected and reconnected. And yes, you can safely apply electrical-rated dielectric grease to most electrical/electronic contacts after cleaning, regardless of the operating voltage.
Some electrical contacts are made of steel plated with brass, bronze, copper, tin, and/or other materials. A magnet is a quick and simple way to check. If such a contact has corroded down to the steel (i.e. there is visible red rust), then it's usually best to just replace the individual contact or the whole connector.
[This message has been edited by Marvin McInnis (edited 05-13-2008).]