I believe that most difficult clients are difficult because the process is more complicated than they expected and therefore frustrating. The best way to counter this is to be open and thorough early on, setting realistic expectations and timeframes. Then keep the channels of communication open so that the client feels a part of the process, and not that they are waiting on you to finish something. Something as simple as explaining why you need a document not only helps explain where in the process things are, but also suggests that everything is proceeding as anticipated.