Once you've decided that process modeling is the best approach to capturing requirements for your project, you don't need much to ensure success: just make sure you involve the right people and use the right tools.
Involving the right people. To analyze a process effectively,
reach out to everyone involved in the process, so you can develop an accurate process model. If several people are involved in a given process function, identify at least one person who can speak for all of them. Use your best judgment, however; having too many people in the room might slow down requirements elicitation, but having too few people (or worse, the wrong people) can bring it to a standstill if no one can answer questions that are raised. Overlooking key participants can also send you down the wrong path if something gets missed.
When you're eliciting process details, include someone who actually executes the process as part of their job. Of course, it's also useful to involve the process owner in conversations about possible process changes or measurements, since they have ultimate authority and accountability for the process. However, the process "owner" may be removed from the day-to-day work, so it's best not to rely on them for the same level of detail you'd expect to get from a process "doer."
Using the right tools. Keep in mind that "the right tools" doesn't necessarily mean you need a robust tool for automated process modeling. While automated tools can help, they often require some setup and training. If you have access to an automated tool but you aren't well versed in its use, don't waste time fumbling with it while your business stakeholders are in the room. Practice on your own time, and get comfortable with it before using it in meetings. In addition to keeping meetings more efficient and making best use of the attendees' time, using tools you're familiar with means you can concentrate on collecting and documenting information, and makes it less likely that critical information will get lost or distorted.
While you don't need an automated tool, you do need a way to present the process visually. Set up a decent drawing space for your meetings, using common tools like a whiteboard, poster-sized sticky notes, and flip chart pages. You'll also want some smaller sticky notes to represent the steps in the process. (Make sure the glue is reliable, so you can reposition them during the discussion.) Set the stage with a handful of clear and intuitive symbols (start/end points, decision points, steps, connecting arrows, etc.), and make sure the participants clearly understand what the symbols or sticky notes mean.
In addition to facilitating discussion, your meeting notes will be important to generating documentation, so plan ahead for both purposes. A whiteboard is easier to erase and edit as the discussion progresses, but it's less portable. Large sticky notes or flip chart pages are harder to change, but you can easily take them with you after the session. If you use a whiteboard, consider taking a picture with a digital camera or camera phone, rather than trying to transcribe everything. In addition to providing a more permanent reference, it will reduce the chance of transcription errors (or at least increase the ability to double-check for them). If you use sticky notes or flip chart pages, don't draw the connecting arrows until the smaller sticky notes are properly sequenced. This will keep your final charts cleaner, and avoid having to decipher confusing, crisscrossing lines once your memory of the meeting has faded.