Always be alert to your surroundings. Many robbery's occur in broad daylight, so take caution while walking through the city.
Never walk alone or appear to be lost. This makes you an obvious target to thieves.
Don't carry too many valuables on your person at any given time. Make sure you have photo copies in a safe place for all your identification and credit cards. In the event they are stolen, contact the authorities immediately and contact your credit card company to report the card as stolen.
Ask locals and/or guides about places to stay away from and places that are relatively safe. Don't just wonder around looking for places to hang out. Ask before heading out and get exact directions.