Anyone with a sense of history is grateful for the modern washing machine.
With very few exceptions, you can always get your clothes clean in cold water. About 90% of the energy cost in washing clothes is in heating the water for hot-water loads. Here's a real tip, though: always use laundry detergent formulated for cold water to get your clothes really clean.
Don't use the extra rinse cycle many newer washing machines offer to get extra soap out of the clothes. If you have this problem, don't rinse the clothes again — use less soap!
And if the weather and your homeowner's association cooperate, serious consider hanging your clothes up to dry outside. The best way to save energy on your laundry is not to use any at all!