First of all, make sure that the sunglasses are properly stored when you are not wearing them, so as to prevent the need for frequent cleanings.
When cleaning your sunglasses, use clean water with soap. Remind not to wipe sunglasses down when they are dry, otherwise you may scratch and damage the lenses or frames.
Pick a very soft cotton cloth that is not frequently used, but it would be better if you have a soft optical cloth.
You may buy sunglass care and cleaning kits in most eyewear stores. A sunglasses cleaning kit usually includes a bottle of cleaning solution, a soft cotton cloth and a carrying box.