The main drawback with touchscreens is that they get dirty much faster and more often. A dirty screen will not only look bad, but will also shorten response time. Therefore, it is super important to clean them regularly. You should use a lint-free cloth, slightly moistened with water (preferably distilled).
To get the best results you should use a cleaning product, usually in the form of a spray. If you decide to buy a spray, you must always make sure you spray it directly on the cloth, never on the screen. You can also easily make your own cleaning solution using 50% distilled water and 50% isopropyl alcohol.