My friend claims to have developed a keen understanding of how the average customers mind works. From his understanding, he's been able to develop two major approaches:

1) avoiding a high profile
1a) the "innocent" flight cover: don't line-up with flights from Paris, Hong Kong or other shoppers paradis' (wait for an "innocent" flight like those from any eastern european country.
1b) the non-US-resident shift: much more US-residents are searched than non-US-residents. If my friend travels with a non-US-resident my friend's friend will bring in ...
1c) The sole-business-flyer masquerade: when my friend travels with his family, he, dressed in his normal business clothes, takes the more questionable items in a piece of carry-on luggage and goes through customs first. A few minutes later the rest of the family follows with the trolley full of harmless luggage.

2) Misdirecting the attention:
2b) The least-favored bag: at costums, my friend puts the bag he doen't want to be inspected up on the platform and leaves the other "innocent" ones on the ground. If the insector searches at all, he invariably picks the one he thinks my friend is trying to keep from him, the one on the ground. Purely as a game, try out for yourself - it is amazing that customs officers could fall for such simple child psychologie.
2b) the duty-free liquor bag is practically never searched.
2c) the wrapping trick: my friend wraps soiled, foreign-purchased clothes around dut-free liquor bottles in his suitcase. The costums officers are so interested in discovering what's wrapped up inside that they miss the wrapping.
2d) the lost-bag trick: my friend had $800-worth of items bringing in. He made two bags (each worth $400), at baggage claim he left one bag on the wrong carrousel. After costums he complained to the airline about his lost bag and got delivered a day later.