Fill one bucket up with soap+water the other bucket with just water. Dip your wash mitt into soap bucket and apply to car. Once you are out of soapy water on the mitt, dip it into the water bucket to rinse dirt off of the mitt. Repeat. This helps to prevent you from washing your car with dirty water.
Also, people like to get grit guards for the buckets. This keeps dirt at the bottom of the buckets and off of your wash mitt.