Couple of things you can do... the filter is obviously ideal, but thats big $$$ and the cartridges need to be replaced often so it keeps getting more expensive over time.
Change the time of day you wash - wash early in the morning or later in the afternoon when the sun is lower in the sky and the heat is less intense.
Work in shade whenever possible - a garage, a tree, even an inexpensive pop up shelter. Anything that can give you some relief from direct sunlight to slow the water from drying.
Pooling aka Sheet Rinse - take the nozzle off the end of your hose and use a slow flow of water to 'push' the excess water off as your final rinse. Water drops are needed to make water spots, if you get most of the standing water off you don't have to worry about it as much.
Choose a soap that suspends minerals - our
Extreme Suds Balanced Wash is pH neutral and will help keep the stuff in your water suspended. Part of this means DO NOT RINSE as hard as it is to resist. Soap up every panel then rinse, dried soap is easier to remove as it keeps the minerals suspended.
Use a drying aid - like others mentioned, use a detail spray. Our
Polymer Detail Spray features an acrylic polymer that adds protection and gloss and is super slick. It helps keep the water 'wet' until you can remove it.
Forced air drying - use an electric leaf blower, air compressor, or a dedicated car dryer to blow the excess water off quickly before it can spot.
A lot of these techniques are outlined in this video, even though it involves a pressure washer the process is still pretty much the same.
[video=youtube_share;_tFbDj7mT2c]http://youtu.be/_tFbDj7mT2c[/video]
Another option all together is to do away with the water exposure and move to a Rinse-Free Wash. Still very effective, but b/c it doesn't require rinsing and running water the issues with water spots are virtually zero -
SCG Pro Rinse-Free Wash & Wax