let's be clear about this: I would never smuggle and I don't want you to do it. These are just stories of a good friend of mine. He uses the following tricks:

1) the wear-and-tear-ploy: you only have to declare the "fair retail value", which should not be higher at which a secondhand store could sell the item (50% off).
2) The per-person-spread: sometimes, when travelling with his family, my friend will pass immigration separately (with an immigration form for each family-member) and then pass customs also separately, letting the familiy members carry "problematic" items in small bags - he follows with the big (unproblematic) bags seperately.
3) the foreign-receipt fraud: when my friend buys in foreign countries the shopkeepers are happy to give him a receipt for a lower amount.
4) The domestic receipt fraud: my friend always has receipts from shopping at home to "prove" customs that he bought the item already before leaving home.
5) the domestic label scam: my friend takes labels from his home-clothing and sews them into his foreign clothing purchases.
6) the previous trip story: hopefully he can prove it with his passport and he may have produced the shop-keeper to predate the receipt accordingly
7) the sloppy list: my friend will make an almost illegible list of foreign purchases including all items in one column but leaving off one of the (higher) prices in the other column. At the bottom of the prices-colum he puts the (wrong) total. A custom officer searching his bag will find all items listed, but may miss that not every item has a price. If he does, he'll probably think it's an understandable mistake.

one more tip of my friend: he leaves one bag at the caroussel (and declares it as lost bag) and passes custom with one bag (which is just under the declaration-limit).

Later, he declares truethfully all items of the second bag (which totals just under the declaration-limit).

this (should have been) is /was tip #22.

and this is also from "the Airline Passengers Guerilla Handbook", Strategies & Tactics for Beating the Air Travel System, The Blakes Publishing Group, ISBN 0-924022-04-3, 1989. It is no longer available: that's the main reason why I pass over some of the tips I found.

hen I travel abroad, I buy very few items...may $30 worth, so I have never had to deal with import taxes, but I have a question. What if I am in a 3rd world country and buy something for the U.S. equivalent of $100, but in the U.S., a similar item might sell for $1000? Do I declare the amount I paid or the amount it might be worth if I were to sell it in a retail store?

A person also shouldn't have to prove that they had already bought something prior to departure of the trip they are returning from; it's almost as bad as those 3rd world "fines" for minor infractions that are really acts of extortion.

Declare the amount you paid for it.

But if you're shipping it, make certain it's insured for the amount it would cost to replace where you live.

When visiting some countries you have to declare things going in - eg your laptop, camera, etc, so keep the serial numbers handy. Occasionally they'll check to make certain that you re-export them.