1. Fill out the Global Refund receipt that the store gives you.
2. Take it to the tourist office.
3. Get your tax back in cash.
4. Get the receipt stamped upon your departure.
5. Mail it back in and hope you filled it all out right.
Anyhow. I spent 138 Euros on clothes, got 17 Euros or so back in cash after standing in line at the tourist office for a half hour, ran around the Barcelona airport almost missing my flight for the silly stamp (it is my impression that the tax-free customs guys hide on purpose--you'll see why below), and then spent 2,40 CHF on a stamp to mail the thing back to Spain. Worth it? No way.
Time is money. And unless you're spending thousands of dollars in a foreign country, forget the tax-free thing. It's just not worth the time and stress.
And here's the catch: my sister paid it. If you fail to get your receipt stamped at customs and fail to send it back after the fact, you are charged. A lot. My sister got a nice $25 charge on her credit card two months after the fact, even though she originally only got 10 Euros of tax money back. So the whole transaction ended up with her having a loss of about $15. (Spain told her Switzerland would stamp her receipt before her flight back to the U.S., but Switzerland wouldn't stamp it because they are not in the EU.)
Lesson: tax free, don't bother. Unless you are spending at least $1000 and are 100% sure of the rules and how to follow through--especially if your trip involves multiple countries that may or may not be in the EU.
Do you have experience with tax-free shopping? Do you think the time is worth the money?