I think, really, that its like any other business -- except you have the "location, location, location!" part already answered.
Identify a need that isn't being adequately met, execute it fully to requisite standards (its not good enough to just be the best of bad options), price it attractively -- get good value for your own time invested, but remove all question in the potential customer's mind whether they could do it better, or cheaper, or both. I forget the name of it, but there's a control-mapping component that's very popular, and I think Unity actually acquired/invested in them. Here at Microsoft, we recently bought the company that created (and successfully sold) the UnityVS plugin (which integrates Unity with Visual Studio for scripting) so that we can give it away for free to help people make more and better games for Windows platforms using Unity. Those are examples of lucrative, core needs that someone decided to meet and made a good living at it.
Now, as a seller without a professional pedigree, part of identifying opportunities that aren't being met is also being honest about which ones you can fully and successfully execute on. Do only the things you can do well, don't pass yourself off as more capable than you really are. That's how bad reputations are born, and nothing kills person-to-person business like a bad reputation.
Another avenue, if you work on your own projects, is to monetize the components and assets you might have made for your own work, perhaps some time after you release your project so that there aren't a bunch of games that look like yours or have your systems.