For modern lenses I use a Lens Pens after going over it with a Camel Hair Brush and it works very well. The Lens Pen often leaves smears that you can then easily get rid of with just your breath and a dry microfiber cloth. If your lens is really dirty I would use a very small amount of liquid cleaner. A drop of Windex w/ammonia works well and gets things very clean. It won't take the coating off modern lenses- but I wouldn't try it with a very old lens.