Synthetic release agents

People who make casting resins also tend to make specific release agents to help you get that resin out of your molds.

Different sythentic release agents work in different ways. Some of them basically coat the mold in a thin layer of silicone or Teflon. The resin won't be able to stick to those chemicals. Other release agents form a thin skin of vinyl. The vinyl will stick to the cast but won't stick to the mold.

Of course, using this stuff is no guarantee that your cast won't stick to the mold anyway. But it does help push the odds in your favor.