This is the million dollar question ;-)
In my experience..
And then some more belief and luck wouldn't go amiss.
Edit: in fact, I feel should elaborate a little more. @deviantony's suggestion of building a minimum viable product is a must whether you believe whole heartedly that the idea will be a hit or whether you just don't know. Building an MVP not only allows you to get the idea out there in the wild and to see how your potential users react - but it also allows you to put a cap on the amount of time and effort you put into an idea that isn't a hit.