A towel re-use program can be approached from a few directions.
The most common way for guests to let you know they don't want their towels changed is to fold them and hang them up on the towel bar. Unfortunately, housekeeping too often doesn't respect the guests' wishes and changes the towels anyway. It's not uncommon to hear comments about how useless it is to ask for towels not to be changed because the request is ignored. What a way to undermine your environmental message!
Hotels commonly tell guests to leave their towels on the floor, in the tub, or in a provided basket when they want fresh towels. What a great way to handle towel changes. If staff is trained to honor the message sent by your guests, the system will work well and guests will respect your environmental commitment.
Another approach is to just have a policy for the frequency you change towels. Some guests will speak up if they want their towels changed more frequently, but most will just go along with your plans.
What is your towel re-use program like? Is your staff trained to honor your environmental commitment? Remember, not washing and drying towels daily saves water, electricity, soap, and labor, and it reduces the wear and tear on towels so that you don't have to replace them as frequently.