This is a great question. We use a models based approach called people systems data (PSD). It divides the models into models that look at the project from the people who are stakeholders or who are using it, the systems themselves and the data involved. The really hard part is capturing all the business rules. When you think of requirements most people dont distinguish very well between actions the system can take and then the business rules that need to be configured. When you are implementing a software package a majority of what you are doing is configuration to implement business rules. There are a variety of ways to get the business rules, what we do is use process flows and then map business rules against very detailed process flows. The business rules also map to requirements as well. This is a huge area so it might help if you have specific questions.

You can go to our blog and we have some articles on requirements for migration projects

http://requirements.seilevel.com/blog/?s=migration