A great way to prevent water spots is to wash and dry in the shade. Either that or invest a ton of money into a filter to get all the minerals out of the water. Deionized water is good for rinses also.
In regards to washing there is so much misinformation out there. Cars need to be washed and if they're washed correctly then no harm will come to the paint. Using a duster to clean it and a detail spray to finish it will result in micro marring over time. It won't cause swirls like you'll see with a rotary buffer and wool pad but if you shine a light on it at the right angle, you'll see tons of little scratches. I usually take about an hour (sometimes more) to wash my car. Some tools include.. 3 buckets (2 for paint, 1 for wheels), grit guards for the buckets, a foam gun, wheel well brush, wheel spoke/barrel brush, lug nut brush, wheel wash mitt, paint wash mitt, paint shampoo, and various sprays and cleaners. I use waffle weave towels and spray wax (Meguiar's D156) to dry and a 420GSM dual pile towel for my final inspection.
I could go very in depth on correctly washing cars but i'll save that for another time. :D