Per a recent episode of "Gardening by the Yard", "To remove the salt deposits that form on clay pots, combine equal parts white vinegar, rubbing alcohol and water in a spray bottle. Apply the mixture to the pot and scrub with a plastic brush. Let the pot dry before you plant anything in it."

Hope this helps.

A good way to ensure that there are no diseases that are left behind in the pots is to soak them 10 parts water and 1 part bleach solution over night, does the trick everytime. Just make sure that after the soak the clay pots in the solution you soak them again in pure water to get the rest of the bleach out and then let air dry. I keep doing this til I can't smell any more bleach on the pots. Or if you don't mind stick them in the dishwasher, I've done it!

After removing any loose dirt, wash the pots in your dishwasher. The deposits come right off.