Have a change - small notes
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.
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.

We having reports from many tourists that Prague Taxi drivers are happy when they can accept foreign currency. Not because they need it but it is a chance for them to rip you off. So be aware and keep in mind that 1 Pound is not same like 1 CZK .. It seems that many taxi drivers there don't want to know exchange rates.