Who are 'difficult' clients? I would say they would be clients with some of these qualities:
a. Those who are never satisfied with the results in your case, even when you have got good results
b. Those who have gone to or continue to take advice from several different lawyers
c. Those who always haggle over the fees, despite being able to pay
d. who have unrealistic expectations about the outcome of the case
I found some interesting resources on handling difficult clients, which were quite useful. However well you may out in systems to deal with such clients - such as proper and clear billing terms and terms of engagement, returning calls, ensuring that all instructions are in writing etc., there finally comes a time when you may have to take the call as to whether to continue handling their matters or retiring from the case. As much as I was feeling quite bad about asking my client to go to another lawyer (as her case was almost going to be settled outside court) there was no way I could continue representing her, as every interaction with her was just so upsetting and antagonistic. When things get to such a level, to remain sane and to continue to be able to love your job you need to take this difficult step.
