I use a Quantray microfiber lens cleaning cloth. The same one I use on my digital SLR camera lenses. It works great for dust and light stains...dog slobber, milk, iced tea and (dont ask) strawberry jam.
They are availible at most camera stores, if you can't find a Quantray...any good major lens manufacturer should have one (Nikon, Sigma, Tamron, ect). Or go to www.adorama.com