Fill the dirty water tank with hot water and giving the tank a good shake then pour the water out. Continue to do this until the water coming out of the dirty side of the tank is clean.

The tech support at I-Robot told me to fill both tanks with salt water, (the same amount you would gargle with) and shake as hard as you can (in your bathtub to avoid a cleanup). I just did this and had huge dirty hairballs come out and don't even own a pet.