Find the cold wash setting
Most modern washing machines have some form of cold wash, it may be listed as the "delicates" or "quick wash" setting.
If there's no specific cold water setting, try switching to the lowest temperature you can find.
Check your detergent
A lot of modern detergents are designed to work just as well at lower temperatures, switching to a cold wash should have very little impact on the cleaning of your clothes.
Shorter, colder washes, will help drastically reduce the amount of energy you use to wash your clothes.
Higher temperature washes should be reserved for heavly soiled items, bedding, and towels. Everything else will be just fine on a cold wash.
Why is this important?
According to Ariel, "choosing a wash temperature of 30°C... can save 57% on running costs, when compared to washing at 40°C" - for lightly soiled clothes reducing the temperature will have little to no difference on the cleaning result so it makes complete sense to make the switch.
As well as reducing the temperature there are other things you can do to help increase the efficiency of your weekly wash, this guide from energy star has some extra tips for you to try on the next wash.