Whether you're licensing photos through an agency or you're working with local businesses, destination photos sell. If you're just starting out, consider uploading your best work to sites like 500px, where you can offer your images for license quite easily. If you already have a strong portfolio of work, consider applying to Stocksy, the most reputable agency in the stock photo business. Interested in reportage? You can sign up for a Demotix account to get started in travel photojournalism.
Photographers for National Geographic spend a lot of time doing research. This helps us figure out what's there—what the place is about and what subjects we need to cover. Read brochures and travel books. Go to libraries, bookstores, or onto the Web. Talk to friends who have been there. Pick up travel information at the country's embassy. Find whatever you can that is relevant, and devour it.
Make use of people to give your images life and scale. If the facade of a particular building appeals to you, the picture may be that much better if you show people walking in front of it. They will give it scale and also let viewers know what sorts of people live there, how they dress, and the like. An outdoor café may be more interesting crowded with people than empty.