When in Rome
When travelling, it's essential to remember that you are a visitor in a foreign land, and it is imperative to be polite and respectful to culture and customs. In Muslim countries you should be particularly careful, as it is strictly taboo to photograph Islamic women or any area or building that could be deemed a site of national security.
The best approach is to ask subjects if they mind being photographed before doing so. If they are nervous or suspicious of the camera, they may become more receptive to being photographed if you allow them to look at your camera, even take a photograph themselves. This often does the trick - subjects become much more comfortable and allow you to take countless photographs of them once they have familiarised themselves with the camera.
Interacting with people and gaining their trust will often encourage them to pose for you, but if they are adamant that they don't want to be photographed, don't push the issue - this can be offensive and inappropriate, and the last thing you want to do as a traveller is make people uncomfortable in their own country.
It is not advisable to pay locals to pose for you - this encourages begging and corruption, and people who are accustomed to being bribed tend to be make far less appealing and natural subjects.
An efficient and knowledgeable guide can significantly alter your travel experience for the better, and can help you to get fabulous pictures by leading you to special places or exposing you to interesting cultural or local situations.