Material that can be cut / carved and doesn't react with silicone
Cover the object in petroleum jelly (aka Vaseline) first, you may also try adding paint thinner to it, to make the coating smoother. 

Using different types of construction silicone may give different results. Some of these contain glue and other components that make the cured silicone stick better to different surfaces. So I guess the simplest and cheapest neutral silicone is the best.

EDIT: some say that adding the corn starch to silicone makes the molds dry faster. Not sure if it makes the silicone less liquid though.