I've found that there's good money in every industry. You just might need to dig deeper for it in some. The best way to narrow your target market is to focus on what you enjoy the most. If don't enjoy the market, you're not going to do your best work. I've turned down clients that offer services or products I have no real interest in because I know I'm not going to be able to fake my excitement about a product or a project. If you do what you enjoy, even the failures are a blessing.