My approach is this: listen for the bitching. People love to bitch. They bitch about what they want that they don't have. They bitch about what they have that isn't good enough. If you listen, you can identify targets. That means either making something that few or no competitors are making, doing it better than what's already out there, or both.
One way I do this is by mining fanfic. I look at what people are writing to identify what they love so much they'll make more of their own, what they wish writers were writing that they aren't, and where they feel compelled to fix things the canon did wrong. Then I load the results into my original canons. It is a goal of mine to write well enough that my fans don't fell compelled to run along behind me fixing the fuckups. So far, it seems to be working.
Another thing I do is read other people's blogs. When I see someone complaining that there isn't enough X or all the X sucks, I say, "Come to the Poetry Fishbowl or the Creative Jam, and I'll write you a thingie!" In fact, just seeing people talking about lack-of-thingie is enough to make me check my records. Often I have indeed already written at least one thingie, but if I can't find any, it goes on my to-be-written list.
And of course, I get a lot of ideas direct from my audience during the prompt calls. That's how I have a whole series about autistic characters, and live happy lesbians, and superheroes who deal in problems that can't be solved by hitting.
That's just me, but the same principle applies to all goods and services. Listen for the bitching. It highlights market opportunities that you can use. If you can meet a customer need, they'll buy from you, and everybody wins.