From the question:
"a lot of conceivable startups have already been conceived, a lot of workable business ideas have already been worked out"
That's like saying a lot of music has already been written. Of course it has, but there's always another song. And there's always another new solution to the world's problems.
Start writing down every idea you have. Every idea. Even if it's not something you think you could even solve. The point is to get yourself in the habit of writing down ideas. Once they are written down, you can research and refine them.
I use the notepad on my iPhone because I have it with me all the time. Sometimes I spend time just thinking about ideas, but most of the time the ideas pop up in the course of my normal activities. I've trained myself to listen and be aware of what people are saying and look for ways I can alleviate pain.
I regularly review my list of ideas. Many of them have already been done, but that isn't a reason for discouragement. It proves that I have great ideas! Someone I follow on Twitter just posted about his need for what one of the ideas already on my list solves. That idea is getting moved up!
One of my ideas would take millions of dollars to pull off. That one is probably dropping to the bottom of the list.
I am currently working on launching and/or prototyping 3 of my ideas. I think my problem now is that I have too many good ideas and I don't have the capacity to pursue them all.