Don't Offend the Locals
Especially when you're in a foreign country, be conscious of local photo etiquette. You might want to shoot a picture of a merchant in a market, for instance, but it's the polite (and safe) thing to ask permission first. Sometimes locals will request payment--in that case, keep moving. Few photo ops are worth the risk of exchanging money in public. Also, be aware of places where photography is prohibited; in general, avoid shooting pictures of military and government installations.