I worked at Waterway Car Wash for a few yearsl. The biggest mistake people make when "hand washing" their vehicle is not bringing a towel or chamois with them to dry it off after.
Preferably, dry it in the shade....time will be more on your side. Also, having a separate towel for windows is a must, two different surfaces (metal body/glass window) attract and hold dirt and smudges differently. Also, having a towel be a little damp is a good thing when drying, since water is polarized the dampness will help attract the water off surface a little better. Sometimes, when you buy a new towel that is completely dry it will just push the water around and create streaks.
If you're up for spending 12-15 bucks, go find a California Dry Blade at your auto parts store...they're pretty good at eliminating water spots....and you don't need to dry as much.