Prepare to soak your clothing, if you've got the time or if you have a lot of clothing impacted by hard water stains. Fill up a washing machine with hot water and quadruple the amount of liquid laundry detergent that you would normally use, but don't freak out about the amount. Check the clothing labels to be sure that the clothing can withstand hot water, then add the clothing to the washing machine.
Measure out a cup of water softener and add it to the soaking clothes. Don't turn the washer on or start a cycle, just let the clothing soak up the detergent-loaded, water-softened water for at least eight hours. Set the washer to drain the liquid after the eight hours are up, but don't allow the washer to agitate all of that detergent or you could be covered in suds!
Keep the clothes in the washer and run it through a cleaning cycle, but don't add detergent. Instead, pop in another cupful of water softener to combat the hard water so you don't restain your freshly cleaned clothes. After washing, if your clothing feels a little sudsy or slippery, repeat the rinsing process.
Make your iron do double duty to remove hard water stains, if you're short on time or just have one article of clothing that is stained. Spread out a clean, white towel and turn on your iron to the setting suggested on the clothing label. Moisten the stained area using a wet sponge, then place the clothing with the wet side down onto the clean towel. Iron over the wet area to transfer the yucky stain from your cute clothes to the formerly clean towel.
Follow the manufacturer's clothing care recommendations, which can be found on the clothing label.
If you have a stubborn stain, or a very valuable piece of clothing, a dry cleaner can professionally remove the hard water stain to make your life easier.