Stored Procedures and the Optimizer

I have been playing with stored procedures (on Informix 7.11 UC1 running on HP/UX 9.04) and have run into a problem I'm not sure how to solve. My procedure includes a 'select' statement that accesses our invoice table and links it to a few other tables. At any one time I want to access the current month's data so I am selecting 'WHERE invoice_date BETWEEN 3/1/96 AND 3/31/96'. This works fine when I have the date values hard-coded at the time I create the procedure. The procedure is optimized correctly and runs in about 30 seconds. However, I want to pass in the date value dynamically. When I write the procedure to use variables for the date values (WHERE invoice_date BETWEEN beg_date AND end_date) the sql is optimized differently and the same procedure takes an hour to run with the same date range. Is there a good way to have this part of the procedure optimized at run time (when it knows the dates)? Or do you know another way to get this optimized the way I want? I have thought of having the stored procedure create a stored procedure, but even if that's possible, it seems the code would be really klunky. Thanks for any info.