How do I auto populate a custom field (pick list) at the Opportunity Product line-item level based on the product that is selected.  That is, when I "Add Product" to an Opportunity, I would like two fields at the line item level to be already pre selected.  How would I go about accomplishing this?  I am using the professional edition.

How do I auto populate a custom field (pick list) at the Opportunity Product line-item level based on the product that is selected.  That is, when I "Add Product" to an Opportunity, I would like two fields at the line item level to be already pre selected.  How would I go about accomplishing this?  I am using the professional edition.