Fast glass is a good product,but its all down to technque.

Find a good quality microfibre,dont use one that you have used on the dash with other products on it.

Some people swear by using paper towels but i cant get on with them for some reason.

Best way i have found is to spray a couple of squirts onto the cloth not the window,now how much you use depends on a lot of things including the air temp and how dirty the window is.

Then flip the microfibre towel over and buff with the dry side ( this is were a crap cloth with leave fibres)