Canvas ballet slippers can be washed in cool water on the gentle cycle of your washing machine. Use mild detergent, and never ever put them in the dryer because they will shrink.
Once you've washed canvas ballet slippers in the washing machine, wear them around the house while they dry. This will keep them from shrinking and will mold them to your feet.
Leather ballet slippers should not be washed in the washing machine. They will become hard and brittle with that much exposure to water. Instead, wash them with a clean cloth and soft soap such as Woolite or dishwashing soap. Another soap that works well is Zote soap, which is pink and comes in cake form. You can find it on the laundry aisle at the grocery store.
If your leather ballet slippers have a greasy spot or stain, make a paste of water and baking soda, and rub it in with a soft toothbrush. Then wipe the paste off with a wet cloth.
Satin ballet slippers are trickier to clean because most treatments will dull the satin. Some people claim that rubbing Calamine lotion into a stain will help, but this will probably dull the satin. For best results, try using a dry soft toothbrush to loosen the stain as soon as possible. Then use a clean wet cloth to try and extract the rest.