Have "next review" be the actual date we schedule with the client for his or her next review -
Have a "next review due" for when they should be due for their next review
Have a "last review" field as well
Have these right where the "next review" currently is so if a client calls in we can quickly retrieve this information. Currently, when we create an activity for his or her review we have to locate that activity (especially if it's planned well in advance)
Then we can easily export and see who is due for a review