You can try the Hot Water Method.  This was taken from the old MoRich site:

The one method we really like is a simple "Hot Water Bath." This procedure helps to make sure that the ball is not subjected to any extreme temperature changes that could cause the cover to crack or separate from the core. Though the process is easy, don't rush the steps! So here we go...

1. Have the ball wet sanded to about 400-grit to open the cover's pores.
2. Fill a tub or bucket (5 gallon buckets work well) with hot tap water and about 2-3 teaspoons of Dawn dish detergent.
3. Wash the ball using a wash cloth or a scotch brite pad (burgundy or green) for a few minutes.
4. Remove the ball from the soapy water and rinse the tub (or bucket) clean and refill it with hot water (no soap).
5. Place the ball in the water and wash it clean with a clean cloth or new scotch brite pad. You will probably notice that a soapy film will appear in the water. This is residue that was trapped in the coverstock (much like the oil and dirt was).
6. Repeat steps 4 and 5 until no soapy residue remains, This make 2 or 3 times to achieve, depending upon the amount of soap that was used. You don't want to use too much soap, but you need enough to cut through the oil and dirt.
7. After the soap has been completely removed from the coverstock, allow the ball to air dry at room temperature.
8. Have the ball wet sanded with 400-grit paper and then follow the steps to bring it back to its factory finish as described in our resurfacing section.

Once this procedure has been completed, make every attempt to clean the ball after each session of use.

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Another method that many on this site recommend or use themselves is the Dishwasher Method.  Put your ball in the dishwasher and DO NOT use the heating cycle.  You only want the hot water and soap to clean the ball.  The heat cycle may not be properly regulated.  Others with experience may chime in on this method.