Crime rates are relatively low in Prague, but travelers should be wary of pickpockets in crowded areas, especially on metros and trams, and at railway stations. Trams popular with tourists—like No. 22, which circumnavigates most of the major sites—are also popular with pickpockets. In general, always keep your valuables on your person; purses, backpacks, or cameras are easy targets if they are hung on or placed next to chairs.
Violent crime is extremely rare, and you shouldn't experience any problems of this sort. That said, you should certainly take the typical precautions you would take in any large city.
Although nothing is likely to happen, it is not wise for a woman to go alone to a bar or nightclub or to wander the streets late at night. When traveling by train at night, seek out compartments that are well populated.
As with any city popular with tourists, Prague has its share of scams. The most common rip-offs are dishonest taxi drivers, pickpockets in trams and on the Metro, and the ubiquitous offers to "change money" on the street. All these are easily avoided if you take precautions. If you have to hail a cab on the street, ask the driver what the approximate fare will be before you get in (if he can't tell you, that's a bad sign), and ask for a receipt (paragon) at the end of the ride. In trams and on the Metro watch your valuables carefully. And never exchange money on the street unless you want to end up with a handful of fake and worthless bills.
Distribute your cash, credit cards, IDs, and other valuables between a deep front pocket, an inside jacket or vest pocket, and a hidden money pouch. Don't reach for the money pouch once you're in public.