Like Gilda,
I just pop those plastic pots into the dishwasher,
otherwise I let them sit a day in 10% bleach solution.
I do reuse clay as a cache pot
without even rinsing,
but am not afraid to soak in bleach soln. if they're scummy looking.
I don't happen to reuse clay on plants directly,
as I mostly move out of clay and into plastic.