Look for a clothes dryer with a moisture sensor that automatically shuts off the machine when your clothes are dry. Not only will this save energy, it will reduce the wear and tear on clothes from over-drying.
The best dryers have moisture sensors in the drum for sensing dryness, while most only infer dryness by sensing the temperature of the exhaust air. Compared with timed drying, you can save about 10% with a temperature sensing control, and 15% with a moisture sensing control.
Look for a dryer with a cycle that includes a cool-down period, sometimes known as a "perma-press" cycle. In the last few minutes of the cycle, cool air, rather than heated air, is blown through the tumbling clothes to complete the drying process.
Gas dryers are less expensive to operate than electric dryers. The cost of drying a typical load of laundry in an electric dryer is 30-40 cents compared to 15-20 cents in a gas dryer.