A sheet re-use program can be approached from a few directions.
The most common method is for guests to put a tent card on their bed when they don't want their sheets changed. Unfortunately, housekeeping too often doesn't respect the guests' wishes, and changes the sheets anyway. It's not uncommon to hear comments about how useless it is to ask for sheets not to be changed because the request is ignored. What a way to undermine your environmental message!
The La Fonda Hotel, in Santa Fe, New Mexico, takes the approach that they change their sheets after three days, unless a guest requests it more often. The hotel has door hangars that the guest hangs on the outside of their door requesting a sheet change. Brilliant! It makes it easy for housekeeping to assess the number of beds they need to change that day before even entering a room. That's efficient. This isn't as common an approach, but it seems logical.
Another approach is to just have a policy for how often you change sheets. Some guests may request more frequent changes, but most will just go along with your plans.
What is your sheet re-use program like? Is your staff trained to honor your environmental commitment? Remember, not washing and drying sheets daily saves water, electricity, soap, and labor, and it reduces the wear and tear on sheets so that you don't have to replace them as frequently.