Copying custom field from Potentials to Quotes

I have a custom field in Potentials called "Job Number" which is a numeric field. When a Potential is won I want to then create Sales Order (or Quote) which also has this field so that when I select Potential Name in the Create Sales Order form it automatically fills in the custom "Job Number" field. I've been knocking my head against a brick wall all afternoon trying to wkr this out so would really appreciate some help. TIA.