Quick tip: Count out your Euros aloud while holding them in full view of your taxi driver!!!
I knew the scam about taxi drivers ripping off tourists, and had counted aloud from the back seat. The taxi driver grabbed my money and artfully switched a 20 Euro for a 5 Euro he had in his other hand and quickly held it up and "showed" me I was short. I balked because I knew differently. I had carefully counted out the correct change before we entered the taxi and put it in my pocket. But what are you going to do when a taxi driver is yelling in Spanish and you have a flight to catch? I was forced to pull out my last remaining 20 Euro note hid away in my passport (breakfast money) to pacify the taxi driver.
A way to avoid this scene altogether is to purchase prepaid vouchers for a shared van hotel/airport transfer through InfoHub for about the same cost, or a little higher than an honest taxi. That's what we did upon arrival. The only down side is you have to check into their booth at the airport after you leave baggage claim, and wait up to 45 minutes before they arrive.
Another good reason to purchase a prepaid voucher is the guarantee of the transfer. A no small thing in Madrid. The day we departed and chose to call a taxi from the hotel (to save a few bucks--ha!) we learned that the subways and buses were on strike. So taxi drivers were in high demand...and making a killing that day! From regular fares AND naive tourists like us. We paid $70 for that 10 minute ride...
Apparently, neither strikes nor scams are uncommon. So beware!