Winson,
I usually pull up the reporting from the Edge site for a time period and show the total bytes and total amount saved.
If you can overlay some of that with what you see in a Netflow type collector reporting before and after the WAAS install, that will give you WAN utilization changes.
Keep track of some standard application response times in a spreadsheet/graph is also handy. For more indepth reporting, NetQOS agents are actually integrated into WAAS 4.0.13 and later, so if you have NetQOS in house, WAAS can report into it's SuperAgent for end to end response time reporting.
Hope that give you a start,
Dan