Hi:
First and foremost, understand the maturity of your management. There is the ideal way to go, but, if you strive too much for ideals, most managers will, in one way or another, derail your efforts. Always remember, the primary purpose of most BA managers is not to make money for the company but to be comfortable.
Having stated the above, the best approach is to always have in the forefrount of your mind best practices. You then pragamatically back off of those best practices as political reality dictate. What would be the best practices?
* Formally identify and get confirmation on the project scope: Create a context diagram. Then walkthrough it with subject matter experts for verification. Problem: This topmost down approach is most often beyond what anyone can do - even very senior level BA's. So, what i often do is create lower level data flow diagrams first and then summarize upwards into a context diagram.
* Remember, from an Agile perspective, all that many projects - even complex projects - need requirements wise is a few data flow diagrams, some entity relationship diagrams, and some screen shots. The requirements details can be handled by verbal conversations between the developers.
* Espeically in the first 30 days, avoid the typical mistakes of drowning in an ocean of detail - strive to work at higher levels of abstraction. Also avoid the trap of focusing on the implementation oriented "hows" vs the unchanging business "whats".
Tony