This is absurd to be honest. Now maybe I've always taken security a bit more serious than most, but I remember several dozen complex passwords that I use on a daily basis just fine. Most of them are 14 characters long and contain every type of character you can use on a QWERTY keyboard. I've found though that I cannot remember it until I enter it several times first, but after that it's easy. I can't believe there are so many people coming in here stating that their organization doesn't do these simple things. I assumed that strong passwords were the defacto standard across nearly all organizations today, especially those that deal with important data (law firms, financial firms, etc.). This is extremely troubling to me to be honest. I guess I really, REALLY need to get off my ass, get some certifications done and start doing consulting on stuff like this because there's obviously a huge market out there for the taking.
As to my answer, the $50 bill recommendation was spot on. You have to get them to understand the ramifications of NOT protecting the data and you have to do it in a shocking and very realistic manner. You may even have to make some people very angry to prove the point, but I would think in a law firm that they would at least understand the importance of privacy, which privacy is one of the cornerstones of security. With the advent of Sarbanes-Oxley, the controls that businesses must place on their data is immense and I would think that a law firm of all places would have some insight into that governance.
I also have to question your insistence on telling management something needs to be done without having a plan to lay out to them. Bad stuff to do, especially in a larger, more structured organization. Always, always, always, go in with a plan, even if it's rudimentary and simple, as long as it gives them the full spectrum of what you're wanting to see and what's going to be affected.
In short, you'll simply have to implement and monitor your users to ensure that they are NOT placing their passwords in locations that are easily accessible. A morning walk-through of their work areas before they get into work may be in order. Collect up any of them that you find and report the management as necessary. Make liberal use of e-mail communications to provide them with acceptable methods of password control. Simply put, if they don't want to remember something as simple as 8 random characters, they must not care much for their job or their clients and it's really that simple. That has to be made clear also, that the passwords are not for them, but for the business and their clients, because ultimately that's what the passwords are for.