Putting a quick sitemap together helps tremendously as it gives you a high level view of what you're working with. It also helps you (and the client, stakeholder, etc.) picture how each screen will connect.
I tend to create these before any wireframes or UI design.
This was done in omnigraffle.
Please disregard the duplicate "Add POI" screen that connects to "Customer List" in this photo. That was taken in an earlier iteration and has since been corrected :P