I don't have a general answer, but in my case it really does come from life experience. Let me give you an example, using one of my current projects. I met two friends (about a month ago) for a brewski. Turns out they are golfers. We went to Harding driving range and Lincoln golf course. When we were on the 2nd hole at Lincoln it occurred to me that there are people who would probably like a photo survey of the local golf courses. We have roughly 15,000,000 visitors a year here. So now I'm building an iPhone app that will help tourists plan their golfing forays.
I see opportunities everywhere I look. And so far, I have not had a failure. Not all ideas are reachable, but there are always many that require no investment other than my time and energy. (and always remember: Time is non-renewable. Budget your time very carefully so it is profitable)
I think the most valuable lesson I've learned is that being successful in business is not based on how much money you have. It is much more based on how you allocate your resources. Time is the most important one, because it is non-renewable.
I wish you great success in the future!