You probably wash your dishes under an open tap. This is a huge waste, considering the fact that this method can use up to 20 gallons of water at a time.
Instead, fill the sink up with water (a large bucket or bowl works too) and close your taps. Just refill it when the water starts getting dirty.
By simply closing the taps every time you do the dishes, you could save up to 10 gallons of water.