The best way to clean your PDA’s screen is to use a microfiber cloth, much like you would use to clean a camera lens or a pair of eye glasses. While cotton is slightly abrasive and could scratch your screen, microfiber cloths won’t scratch your PDA’s screen. In addition, it's best to avoid using an air can, as they can also potentially damage your screen.
In addition to cleaning your screen, it's always a good idea to invest in some long-term protection, aka a screen protector. There are many available on the market, some of which are precut to match the size of your PDA’s screen. Most of these protectors aren’t very expensive, but they are very valuable in protecting your screen.