Nothing cries 'tourist' more than waving one's hands vigorously and fruitlessly as off-duty and fare-carrying taxis speed by. Here's the simple rule: if the taxi's numbers on his rooftop are lit, he's free. If they aren't lit, he's not. And if they are flanked by "off duty" lights, he's not. Here's the exception: sometimes an off-duty driver can be convinced to take you to your destination: if it's on his way home. If he stops for you, inquire.
Here's another vehicular tip: those town cars honking at you as you stand on the corner are livery cabs. They are not licensed to carry passengers but have a brisk business. If you are desperate for transportation and air-conditioning, it's an option.