Here are some tips to make your molds dry better.
1. Get the right kind of clay mix. I used to mix my own but its timely so I buy mine from Standard Ceramic Supply Company. Look for clay that is used for high fires. Raku clay can take thermal shock well, stoneware is very durable but not very smooth on the surface unless you get a batch with smaller mesh size of grog.
2. Make sure you wedge the clay well to get all the air bubbles and "floaters" out. Cut it with wire to see if the inside is clean then wedge more. Then slap the clay at an angle downwards towards the table to flatten and thin it out or use a rolling pin. DO NOT GET PLASTER IN YOUR CLAY or it will explode (break) in the kiln firing. If you get plaster on the surface, just scrape it off.
3. I dry my clay on plaster board. It soaks up the moisture well and cover my clay tiles with plastic loosely over the entire piece. I will occasionally lift up the clay off the plaster board and reset it down on a fresh dry board. Just know that clay will shrink as it dries and by lifting it off the surface every while allows for the clay to de-stress and keep from cracking, esp. in the middle.
4. How to cover clay with plastic is important. I will cut a hole in the plastic in the middle or near thicker parts of the clay to allow for them to get more air. The edges will always tend to dry first so I watch them and cover them more closer than I would the middle.
5. You can also place clay slabs, tiles on top of each other and alternate them as they dry to keep from bending and warping. I do the same thing when firing a bisque fire to keep down the warp.
6. Slowly reshape the clays edges and body up until leather hard stage. Once my clay is at leather hard stage, I usually have the plastic very loose on the clay or none.
7. keep the clay away from one-sided air drafts and temps.
8. Remember one thing, you want the clay to dry uniformly. That means the thinner sections get less air and the thicker sections get more.
9. And lastly, you must fast chocolate during the entire drying process so that the great clay elf fiery knows you are serious.
There is a guy on this net named Ted Metz who is a wealth of info. He was my clay/sculpting professor 20 years ago and knows a ton more. He is also very good at metal casting and working. He is still teaching and creating large outdoor sculptures these days.