As tommix said, you need to be sure to cool them quickly. Dunking them in cold water is the most common method.
Another thing I have read is that once cool by dunking in cold water, if you dunk them into boiling water a few times the casings will shrink tight as they cool again. It worked on a batch of snack sticks that I made, but that was the only time I had tested this technique.
Pigs are magical creatures.... they turn vegetables into BACON!!