First of all,clothes dryers do NOT earn the energy star label. Clothes dryers, by nature are very inefficient, akin to the incadescent light bulb. How you use one will make the difference.
If you want to be green with your dryer, then here is the deal:
1. make certain that the duct joints, if any, are sealed tightly.
2. follow recommended maintenance
3. clean out the lint trap regularly
4. go ahead and shock the neighbors and hang your undies outside!
5. install a diverter
A diverter is a valve that you can use to direct the airflow leaving the fryer. In the OUT position, the valve directs the air to the outside of the house. In the other position, the valve allows the air exiting the dryer to return to the home. By doing this, in the colder months you will be using the energy (heat) created by the dryer to put warm moist air back into the home. In other words, you can use it as a supplemental heater!
The diverter valve has a removable lint trap like your dryer to keep the lint out of the air. By returning this unused energy (heat) into a cold house, you may be able cut a few therms off the old utility bill. We have 7 people in our home, so our dryer gets a work out, but the rooms next to the exit valve are always warm.