What kind of molds (wood, metal, plastic?) are you covering? Can you use a mold release instead of plastic bags? Are you trying for smooth sides or are you wire brushing the sides after unmolding? Plastic will always leave lines in your pieces. Billie

I sometimes cut a lg trash bag along the side and end. At least you have a large straight piece without seams. Kathy

You can purchase rolls of plastic in all different weights/thickness. I have used the very cheap / thin... disposal able plastic drop cloths you can buy at places like wally world or the dollar stores. No seams to mess up projects...

Thanks for the input. Billie, I use a variety of molds, usually plastic terra cotta pots, plastic tupperware-style bowls, even a metal bowl set. I historically err on the side of dry mixes (I never made mud pies as a kid, we couldn't afford mud :-) ) so I try to make a wetter mix which gives me a vacuum effect come demolding time. I am using Pam as my demolding agent, but I had a rough time with this last pour. Looks like I have a good suggestion or two for seam-free plastic and I'll try that - along with Pam - for my next pots.

You definitly have to line any metal, terra cotta, ceramic or pottery mold with plastic, a mold release won't work. Plastic can just be sprayed with PAM or wiped with vaseline.
After the piece as set for 24 hours...If you're getting a vacuum and the piece won't release you can drill a small hole in the bottom of your mold. Or if it's a plastic mold place it into a warm water bath or wrap the mold with wet warm towels. This will soften the plastic making it easy to release. Billie