Learn from my mistakes, and best of luck!
I have been driven around in too many figure-eight loops by Greek cab drivers who double my fair if I put anything in their trunk, if it's after midnight, or if I'm going somewhere they don't want to go. One guy even doubled my fair because I was on crutches! "Why? Why?" I kept asking him as he pointed to my wrapped up ankle, as if my disability was weighing down his cab.
Through trial and error, I have decided that the best way to get where you want to go as fast as possible in a Greek taxi is to follow these rules:
1. Pretend you are Greek.
Here are the words you will need: "ya sas" (hello), the name of your desired location in Greek, "ti kanete" (how are you), "areestera" (right), "thexia" (left), and "etho" (here). Using these, you can fake it a little bit. When the cab driver tries to make small talk, just look out the window distantly, like you are thinking of other things.
2. Before you get in a cab, say "poso kanee?" (How much will it cost?).
A good cab driver will give you an estimate but say that the meter will figure it out.
3. Insist that the cab driver starts the meter right away.
You can usually do this by primitively pointing at the meter or tapping on it. If they say no, or insist it will be cheaper if they give you a flat rate, get out of the cab immediately.
4. On the opposite end of the spectrum, you can be a mega tourist and pull out your map in the back seat.
Pay attention to the road signs and make small talk to give the cab driver the idea that you know where you are going. ("Oh! I knew we were going to pass Hadrian's Arch on the way to Syntagma!")
5. Make friends with the driver.
I once offered my cab driver a chocolate bar. If they speak your language, ask them about their life, where they are from, and their family. Ask them where you should go and what you should visit. Use any Greek word you know, as this will generally entertain them.