If you are paying in Euro make sure you have small notes – otherwise you might find that the cab driver doesn’t have enough change. If you are lucky you might get back something in Czech coronas. If you don’t need to … to be honest you may as well take a bus or a tram because it’s fast easy and it’s a lot cheaper.
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.
Generally if you speaks English chances you will get rip off are very high!