How can I create a report that shows this month's sales total by customer, and next to it, last year's sales for the same month?

Example;
May 2014 Sales by customer, May 2013 Sales by customer.
would like to do this in a columnar fashion.

Thanks

you could try
formula1 this month's sales
if year(data.datefield) = year(currentdate) and
month(data.datefield) = month(currentdate) then data.amount else 0

formula2 last year sales
if year(data.datefield) = year(currentdate)-1  and
month(data.datefield) = month(currentdate) then data.amount else 0

group on customer
create 2 summaries
sum(formula1)
sum(formula2)