Fill washers (unless they have a small-load attachment or variable water levels), but do not overload them. In general, washing one large load is more efficient than washing two small loads.
Don’t use too much detergent. Follow the instructions on the box. Oversudsing makes your machine work harder and use more energy. Do not overwash clothes. Delicate clothes don’t need as long a wash cycle as dirty work clothes. Presoak or use a soak cycle when washing heavily soiled garments. You’ll avoid two washings and save energy.