I always have a list of what I bought and the receipts in my hand. Sometimes I have been significantly over and they have waved me through. If I'm going to buy something "controversial", like cheese when I go to England, I look up the tariff codes and rules before I go and print them out. It shows the customs people I have made an effort to know what the rules are and to comply with them.

I had one agent get so engrossed about the 200g of cheddar I bought that she totally forgot to ask what the total I spent was ( which was about 2x the duty free allowance). I had filled out the declaration card with the proper amount and declared it, so I did my part.