I use water and a warm, damp, dark space - mold will start to grow in a few days if you\'re lucky.
Seriously though it depends on what you made the sculpt from - and on what you want to cast it in.
Most common is Room Temperature Vulcanizing rubber - you can find it on the web or in several hobby shops stateside. From what I understand, resin will sap the rubber however - and you have to do some work to make it work right with even the reletively low melting temperature of lead or pewter.