If you speak German
you might get better deal ... Another story about the taxi drivers.... One of my friends approached a taxi driver and asked him to give her a flat rate to get back to our hostel, he quoted her a price and she came back to tell us how much it was. I wanted to try an experiment, so I went out to the same taxi driver and asked him in German how much would he charge for a flat rate fare, he quoted me at HALF the price he originally told my friend. I know that it's a sad and unfair advantage for most travellers, but if you speak German, chances are you will get a much better deal!
Generally if you speaks English chances you will get rip off are very high!
Your tips
If you can try to use reputable AAA TAXI or Profi Taxi.
Be sure you are taking a taxi from some reputable company. Usually they speak English and you are going to pay what is fair. Look at the taximeter and be sure they are charging the right tariff (number 1 is on the day and number 2 on nights) never pay if you notice the taximeter is on tariff 3. You can see the number on the top left in the taximeter. Never trust in Russian drivers, once I had to pay 800 CZK for one single ride inside the city (and he was trying to charge me 1000), you better take another cab.

One of many feedbacks about using taxi in Prague: I have very recently visited Prague and I feel that you have some of the worst taxi drivers in the world. When it comes to making up prices for journeys, we come to see your country not to get ripped off. Will warn everyone I know to be careful when getting taxis.. Avi Singh