Sometimes long distance travel is unavoidable, but just because guests are arriving from out of town doesn't mean you can't find environmentally friendly ways (and places) to accommodate them.
If guests need a place to bunk for the night, encourage them to stay in green hotels or other accommodations that are environmentally conscious. A "green" hotel is a hotel that has environmental issues in mind and is dedicated to preserving and protecting the environment through sustainable practices. Check out the Green Hotels Association for information about participating places to stay. You can also contact hotels directly to find out about their environmental management practices. Some of these could include turning off heating and air conditioning in vacant rooms, avoiding personal-sized toiletries, recycling and offering a program for guests to reuse linens from day to day.
Also be aware of where hotels are located. Accommodations near your venue, or near easily accessible public transportation routes, encourage guests to arrive in ways other than by car.