Use a soft cloth (microfiber or cotton) dampened with water, or some soap solution (liquid hand soap). It should be damp enough to actually wet the screen but not so wet that water will run down the screen. If you have a more stubborn mark then you can try some rubbing isopropyl alcohol on the same type of cloth. Vinegar mixed with water at a concentration of no more than 10% vinegar should also work fine (that's 6 1/2 tablespoons of vinegar per quart of water, 1 1/2 tablespoons per cup, or 100 ml per liter). The wipes sold for cleaning glasses or monitor screens also should be good.
Do not use: Paper towels - they are abrasive. Dish detergent - it has abrasive salts in it. Household cleaners - they can contain damaging chemicals. Acetone Ethyl Alcohol Ethyl Acid Ammonia Methyl Chloride Petroleum Benzene