Any number of things can cause a bad molt and to subsequently prevent it.
Crayfish usually have bad molts due to prior injuries that result in the shell getting stuck (this can be from lost limbs, shell rot, etc). If crayfish lose limbs and they aren't clean tears, going in making a clean cut can prevent that part of the shell from becoming stuck later on. It's not always necessary, so that's a matter of making a personal call. Personally, I'll just wait around with a crayfish that's molting to see if it's getting stuck by the previously damaged area and free it rather than removed damaged parts early on that may or may not have become an issue. But some people do this habitually with great success of preventing future complications during molting.
Shell rot, if present must be treated continually every couple of days until the lesions become bleached out or the next molt occurs, whichever happens first. The more rot there is, the more likely it will get to the under layers and cause the shell to stick during molt. This is a large percentage of what causes bad molts in crayfish.
Tanks with lower oxygen levels can also lead to bad molts, the crayfish actually suffocate during the process and die. If I see a crayfish that looks like it's going to molt, I either add additional aeration or remove it to a tank that I can provide it with more aerated water.
Crayfish that are kept with tankmates are best removed or separated if they show signs of molting. They are easy targets to begin with, but the added stress can actually cause complications for them.
For the most part, bad molts are pretty much a sh*t happens thing. But in some cases, there are things you can do to help prevent or limit them.
