Flip it on its head: don’t start with an idea. Start by identifying a group of people you belong to (e.g. “Rails developers”), because it gives you an instant leg up in knowing their struggles. Then research their struggles. Write about them, and solve them. Grow an audience (read: email list). After all that research you’ll have a solid idea of what they already want and buy, and you can create that for them. Plus you’ll have built up an audience to sell it to!
It’s not necessarily quick but it works. Amy Hoy & Alex Hillman were the ones who taught me this strategy and I heartily recommend reading everything they’ve written at https://stackingthebricks.com :)
It is a lot easier from a technical standpoint if the group you pick is not already in tech. Replace "rails developers" with something like "antiques collectors" or "massage therapists" or "arborists" any other profession that isn't saturated with software options already. These people all have smartphones and computers but probably have only crappy options for how those tools specifically accelerate their work.
It’s not necessarily quick but it works. Amy Hoy & Alex Hillman were the ones who taught me this strategy and I heartily recommend reading everything they’ve written at https://stackingthebricks.com :)