Be careful where you exchange money, especially on Wenceslas Square. Many currency exchange offices use legal but unscrupulous tactics and tack on excessive fees. The best way to change money is simply to use an ATM machine. You will get a slightly better ("middle") rate and the only fees will be from your own bank (be sure to check with your bank before traveling to see what fees you’ll incur). Obviously, DO NOT exchange money through strangers who approach you.
Do not use public transportation without proper ticket validation. Purchase tickets through automated machines in subway/metro stops or at newspaper/magazine stands. Vouchers are good for Prague trams, buses, all metro lines, ferry boats and even the cog railroad to Petrin Tower. Though at times it may seem that you’re the only person doing so, be sure to validate tickets as necessary. No, you are not the only one paying - most Prague residents have long-term passes (up to one year) which do not need constant validation, and pensioners ride for free.
A great way to see the city is by trolley. Purchase a 32 CZK (adult) transfer ticket and ride up to 90 minutes. The best lines to see Prague on are #s 22 and 23. They’ll take you through the historical parts of New Town, across the Vltava river, through Lesser Town and right up to the Prague Castle area. Though the subway is much faster, most any trolley will take you somewhere that’s worth seeing.
Unless you or someone in your party speaks perfect Czech, try to avoid traveling by taxi. Drivers pick up on foreign accents and in many cases tend to charge more. In many instances, the subway system is faster (and much cheaper) than going the same route by taxi.
Keep in mind that Prague is very small. A healthy person can easily access all historical or cultural places of interest simply by walking there.
Have fun! Prague is one of the most beautiful and best preserved cities in the world. With a little knowledge and common sense you’ll garner experiences to be cherished for a lifetime.