Trend question
 

I can run a report that will show me sales by month for a particular product
from the original launch date. The first row looks like: Customer Name |
Month 1 | Month 2| etc... and what I'm trying to find is customers who have
stopped ordering which probably means they're buying from a competitor. How
can I easily recognize which customers have stopped ordering? Here are some
of the problems I run into: customers placed their first order at different
times so they have a zero in each cell for all of the months after the
product launch and before they started ordering and some customers don't
order every month. Most customers should be ordering in a consistent trend
though (might be all the time, might be seasonal etc.) how can I use excel
functions (or maybe a macro) to make some kind of trend index that will show
me when a customer's ordering pattern suddenly stops or changes?

Thanks!