Our towels do get softer and nicer the more you wash them, but if you're impatient we've got a simple, inexpensive solution.
Add 1 cup baking soda in place of detergent and then line dry the towels in the sun if possible. A quick tumble on low heat afterwards will soften them even further.
Also, always use medium or low heat if you machine dry. High heat can cause cotton fibers to tighten, making the fabric feel stiffer.