Richard,
In your examples for shipping and lead source you add an autonumber ID field to be used as a key field. Why not just leave the one field in the table and use that field as the linked field. It is already unique. The up side of this is that you will know the real value of the field no matter which table you are looking at instead of seeing a number that means nothing until you create the relationship.

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Reply from Richard Rost:

Are you suggesting removing the LeadSourceID, for example? Then you're left with just a text value. You don't want to form relationships on a text value. If you want to change, say "Word of Mouth" to say "Referral" later, then you have to change it in all of the related tables too (which, yes, you can do with a Cascade Update, but it's just considered bad database practice).