I think that there are more people wanting to learn photography than there are people that want to buy images. And for that reason, teaching has become a major source of income for a large number of photographers. In fact, some of the world’s highest earning photographers aren’t always the most talented, but the ones who can teach effectively. Lots of photographers sell books, video tutorials, and host workshops to help supplement their income.
The more willing you are to travel at a moment's notice, the more opportunities you can access. Day jobs will limit travel, so will mortgages and car payments. Photojournalist Lynsey Addario recently wrote about being 7 months pregnant while on assignment in Gaza. I deeply admire her bravery and commitment to her work, but I imagine many photographers aren't willing to make such compromises. Consider your lifestyle, and how much time you're willing to spend away from home. As for myself, I'm a long-term digital nomad traveling with a suitcase and a backpack and an open mind. Being available and flexible has made a monumental difference to my career.
Travel photography as a genre is one of the most open in terms of the subjects it covers. Many travel photographers specialise in a particular aspect of photography such as travel portraits, landscape or documentary photography as well as shooting all aspects of travel. Much of today's Travel Photography style is derived from early work in Magazines such as National Geographic magazine from photographers such as Steve McCurry. This genre of photography entails shooting a wide variety of subjects under varied available conditions, e.g. low light photography indoors, available ambient light photography for exteriors of buildings and monuments, shooting on the streets where sometimes conditions may be hostile, capturing moments which rarely recur, capturing the magic of light while shooting landscapes, etc.
For example, a product review I did in December of 2014 led people to the product on Amazon via an affiliate link. That product sold extremely well, and people also bought other things well they were shopping. That affiliate link earned me a pretty penny. Of course, that’s not always sustainable. In January and February I earned $300 and $400 respectively from affiliate sales. In March, I only earned $50.
Another place to capture expressions are the subways; either on the platforms or in the cars themselves. My favorite images are the one of a young woman avidly watching a movie on her smartphone, while wearing a single hair roller to tame her fringe....and of the young girl who appears to be viewing a smart phone screen on an ad on a subway platform while her mother is busy texting on her real phone.
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.