For "hard" water, you could invest in a water softener such as this;http://crspotless.com/
Personally, I'd do the following;
- Wash ONLY when the car is cool.
- NEVER wash in direct sunlight.
- Wash and rinse a panel at a time.
- Get a good coat of wax/polish on the car ASAP. This will be your best protection.
Prior to installing a water softener at home, as long as I followed the above steps, I never had a problem.