I just got back from the 12 Day Rick Steves Tour of Portugal which was fantastic. I arrived in Lisbon two days early and was worried after reading about the warnings of taxi drivers ripping of tourists arriving at the airport, and wasn't sure after a long overnight flight to deal with the airport bus.
I found a great private taxi driver which wasn't much more then the estimate for a regular taxi. She had great reviews on trip advisor, so I contacted her (and had contacted other companies which were more money) She was fantastic, and even said if I needed anything before the tour to call her. It was like being picked up by a friend at the airport
Her name is Margarida Neves and her facebook and twitter pages are below.