Hard towels? This way they become fluffy again even without fabric softener. Vinegar helps against limescale deposits, it can reduce laundry stiffness.
Salt: Simply add a tablespoon of salt to the fabric softeners compartment of your washing machine and start the wash cycle. If you don't have a dryer in your house or would rather save electricity, you should dry your towels in the fresh air. A draft of fresh air also keeps the laundry constantly moving and prevents the towels from coming off the line.