I listened to the first part of .net rocks show 722 whilst out for my lunch. Dan North mentioned the excellent output of developers sitting with traders coding and pushing. To me this is one of the ultimate ways to achieve excellent software. I am a big fan of the idea of method acting as a software developer. I am pretty sure someone else has already coined the idea.
Essentially I like the idea of sitting and working with the people I am going to write the software for. Working with them in their daily tasks as if I were a normal employee. It's how I started out coding. I worked in various jobs as an administrator, a construction estimator and many other roles. Initially with Excel and Access and then with VB.net and other tools I just wrote software that made my particular tasks and those of other people in the office easier. The code was outrageous but generally worked. When it didn't I could fix it easy enough, the person who had the problem told me there and then what was wrong and what they expected it to do.
Sitting with people in this manner can help weedle out the features that the client doesn't even realise they need. I don't believe there is any greater method. Even asking them to do it front of you can cause a user to act as they think they should instead of how they actually work. Working with them builds up trust and a relationship that will help you all the way through the process. Think of the undercover boss idea.
There are problems associated with this obviously.. Your company loses a dev for a little time, and there is the question of paying for the developers time whilst this process is going on. For me the time saved afterwards in development and the strong understanding of the domain that will be gained pay for themselves.