+1 on the microfiber once a month. It works.
I use the rain-x wipes, or armorall glass wipes.
Most of the build up comes from you and your passengers. Breathing, vaporized sweat and tobacco smoke if you happen to smoke.
easier way or what to use??
I remember a long time ago when I was a young Marine in California...
I had a beautiful girlfriend who had parents that lived up in northern California and we'd take trips up there to visit every once and a while. One time we decided to take the coastal route back down to SoCal and take in the view. It was a great time; the sights, the sounds of the ocean and my sweetheart sitting in the seat beside me as I drove her Porsche down the Pacific Coast Highway were the epitome of blissful fun. Somewhere along the way, off the beaten path, we stopped for a break to stretch our legs, breathe in the ocean air and rest. I can't go into all the details, but suffice it to say that before we got back on the road, there was some car fun too, which made it even more memorable. Now the reason I tell you this is because some time later, when the temperature changed with nightfall, I discovered a reminder of the earlier activities of the day... my girlfriend's bare footprints in the condensation of the windshield. The defroster had some effect, but the oil from her naked feet must have created some barrier on the glass. It was distracting on many levels, not the least of which was navigating L.A. traffic with the tangible evidence staring me in the face.
I suspect that Rain-X will solve this problem if you should ever find yourself in the same position. It might also help you to avoid an embarrassing and potentially dangerous situation. I would highly recommend it.
You just can't make that stuff up! 
I use "Clear Glass", I get it at Lowes. It is in a black bottle and works great with no residue and or streaking. I also use a microfiber towel as it cleans better than papertowels or terry cloth towels.
