A simple way in Crystal, I am assuming that the report date is restricted to a single full year, then:
Make a formula extracting the month from the date field - MONTH(Subtotal)
Then make a formula for each month - if {@Month} = 1 then {Subtotal} else 0 - do this for each month.
Alternative if you have an open date on the report, no overall date restriction and your SQL is a DateTime field use:
if {DocDate] in DateTime (200, 01, 01, 00, 00, 00) to DateTime (2009, 01, 31, 00, 00, 00) then {Subtotal}.
If is is a date field use:
if {DocDate] in Date (200, 01, 01) to Date (2009, 01, 31) then {Subtotal}.
Please these calculated fields in the Details section. Then create a Group for the Customers and you can sum the invoices to the Group (Customer) and hide the details.
This method requires all of the data to be pulled into Crystal. If you can create a view on the SQL server to sum the records by Customer or Sales region it is much quicker to only pull the sums in to the Crystal report.
These are simple solutions, but will require an annual update on the formulas, option one on the overall date restriction, option two on each of the monthly formulas.
Hope this helps.