Be cautious in crowded areas and in the poorer areas of large cities. Be wary of anyone stopping you on the street and even in car parks to ask for directions, the time, or where you're from—particularly if there's more than one person and if you have recently visited the bank or an ATM.
There's enough of a police presence in Portugal that women traveling solo are relatively safe. Take normal precautions, though, and avoid dark, empty streets at night. Ask your hotel staff to recommend a reliable cab company, and whenever possible, call for a taxi instead of hailing one on the street at night. Avoid eye contact with unsavory individuals. If such a person approaches you, discourage him politely but firmly by saying, "Por favor, me dê licença" (Excuse me, please) and then walk away with resolve.
Shopkeepers, restaurateurs, and other business owners are generally honest, and credit card receipts are rarely subject to copying. There have been occasional incidents of highway robbery, where the thief slashes the victim's tires during a stop at a gas station and then follows the victim, offering to "help" when the tire goes completely flat. In other cases, the thief takes advantage of an unwary traveler who has left car keys in the ignition or money or a handbag on the seat while stopped at a gas station by telling the driver(s) that they have a puncture in a back tire and urging them to get out of the car to inspect.