I have two different "reports" to forecast results with clients. For my physical organizing clients the before and after picture comparison has the biggest results. It's a reminder to clients where they started and where they finished. It gives them a sense of accomplishment.
As for my virtual assistant clients, we typically take count of measurable items. For example, the number of emails in their inbox, the number of photos, etc. By writing down the number we start with and setting a goal of the number to finish, we are able to track our progress and physcially see the number deplete.