Part of the problem is the quality of the air you live in, if food is ever in the car, and what other products you have in the car. If you use a petroleum based dash shiner or something like that, when parts evaporate your glass will be coated/clouded.
Our Glass Cleaner cuts through those chemicals very easy. Follow with Detail Spray for some shine (yes, even on glass.)
One thing you can also do is clay bar your glass to remove contaminants that you might not see simply because you've gotten used to them being there. Use Brilliant Spray Glaze for a layer of protection on the glass.