A towel re-use program can be approached from a few directions.
The most common way is 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. The problem most typically associated with that approach is that 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 to not 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 the guest, your system works well and respect is gained by guests for your environmental commitment.
Another approach is to just have a policy for the frequency you change towels. There will be guests who are more assertive about what they want and will speak up when they want their towels changed, but most will just go along with your plans.
What is your towel re-use program like? 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. And are your staff trained to honor your environmental commitment?
For the travelers using this site, an interesting way to travel green, and make sure your request to reuse your towel is honored, is to book a hostel rather than a hotel. In most eco-friendly hostels you'd have to ask to have your towel changed. What a great way for a hostel, or hotel, to use less energy to keep their customers satisfied.