I have looked into this on several occasions as my wife buys gems from all over the world to make jewellery. Also the jewelllery making tools are cheaper it the states!
Basically (as was said earlier) if you buy an item under £18 they do not harrass you for import duty or VAT AND if it is marked as a gift this figure increases to £36
If an item is marked as a warranty return then they will require the stamped proof of postage OUT of the country! There is a specific form for this which you get your carrier/post office to stamp THEN when it comes back customs hold the item until you send them the paperwork OR you pay the import duty/vat/admin costs and claim this back using the paperwork!
Avoiding import duty is illegal and you risk loosing the item itself. People have tried and got away with numerous techniques such as:
1 - undervaluation (customs can open your parcel and value it themselves then confiscate it or charge you extra!)
2- marking the item as an engineering sample (again if it has a value and customs think the value is over the limit they may charge you or confiscate it)
3 - removing the item from its box bubble wrapping it and sending it (again risky as postal insurance will not normally cover items not in their original box).
4 - having it sent to a friend in the states and then they re package the item as a
second hand item wrapped in birthday paper!
All in all its a big gamble if you try.
I once had customs asking me questions about an old circuit board from an 8 bit computer sent over with "obsolete computer equipment of no monetary value" - thing is the label was correct !!!
The favourite seems to be just order it if the cost to you is cheaper with 17.5% VAT, 5% import duty and £18 handling charge - This is what TNT/post office/parcel force charge.
Personally I look for the "we repay your import duty" auctions on ebay - I bought £2800 worth of kit from Hong Kong (DIGITALREV: D200,D40x kit,2 flashes and 3 extra lenses) and had to pay import duty but this was claimed back within a few days.