Why go
Prague sightseeing often joins a crowded bridge, central squares and public transport in one day. A few deliberate habits help at those transitions: put your wallet where you can feel it, close your bag before boarding, and pause to read a bill or exchange quote instead of rushing.
The full story
At Charles Bridge and Old Town Square, stop to look around without leaving a bag open or a wallet in a back pocket. Use the same habit on a crowded tram or at a metro station, especially while you are finding a route or ticket. Keep your camera and phone in hand or secured when you are finished using them. These are practical precautions for busy places, not a reason to avoid them. If you need to change money, ask how much Czech currency you will actually receive before handing anything over. Read the quoted rate and any fee together. For a restaurant bill, check unfamiliar charges and ask for an explanation before paying. If someone claiming an official role asks to inspect a ticket or passport and demands money, ask to see identification and seek help from the relevant transport staff or authorities. For current transport, emergency or legal instructions, use the official service rather than relying on an old rule of thumb.