These are my favorite ones; they are a little pricier than others, but their surface is completely smooth. Some of the less expensive ones have a bit of a woven texture to them, which is not ugly, but not always what I want on the sides and bottom of my soaps.
These are very easy to crease and cut as needed to fit your mold. I lightly tape the side pieces to the mold. Some people also tape all the seams together, but I'm too lazy for that. Very little soap leaks between the seams, and it is easy to clean up.
After you unmold the soap, just wipe these down, and they are ready to use the next time. Easy-peasy!
ETA:
these ones are larger and less expensive, but they do have the slight "weave" texture to them.