Though some people recommend using Windex, ethyl alcohol and ammonia-based cleaners can yellow your flat-screen or laptop monitor over time, so it is best to avoid such products. You can find an array of commercial products specifically designed to clean LCD screens, but you may prefer to just head to your kitchen and mix up your own solution. Plain old water can do the trick, or you can concoct a solution of vinegar and water or isopropyl alcohol and water.
Always use a soft cotton cloth, rather than a rag or paper towel to wipe the screen. Some people recommend special electrostatic cloths, like those used to clean eyeglasses treated with anti-glare finish, or even dryer sheets. Apply the cleaning solution to your cloth rather than directly to the screen and wipe in one direction -- from the top of the screen to the bottom.