Taxis in Prague are plentiful. Unfortunately, you should think twice before waving one down in the street if you don’t want to be ridiculously overcharged.
If you need to get one, call the company in advance and negotiate the price, or go to one of the stands where you can see prices on common routes. When inside, make sure that the driver’s ID is displayed, and the taxi meter is at zero.
Alternatively, check out the shuttle services that run between the city centre and the airport for more peace of mind.