Machine wash in warm water, with similar colors.
Use 50% less detergent than you typically would*.
Tumble dry low, with a fabric softener sheet.
*When washing towels, we tend to add more soap than they need. The soap ends up being soaked up by the towels and hardening the fibers. If this happens, use a lot less soap than you usually do and add a fabric softener sheet to the dryer. The towels will come out clean, refreshed, and soft as new.