It is very difficult, especially if you need it to be an instant success (i.e. replace your salary before you are homeless).

I have a project (a new kind of general-purpose data management system) that I have worked on for over 10 years. In the beginning, I hoped it would 'take off' and replace my salary. I was never able to quit my day job because it was so lucrative.

Now I am retired. I still work on it in bursts (spend many hours for a week to get something working, then don't touch it for a month); but I treat it like a hobby.

Maybe it will catch on (it does some amazing things with large data sets), but maybe it won't. I try to spread the word on forums or in my blog, but I am not a big marketing guy and there is so much noise out there that everything gets lost.

Good luck. You will probably need it.