Learned from both Paul Graham's article “How to get startup ideas” and my own experience. You have to read it if you are thinking about starting a startup.
You cannot find great startup ideas by deliberately searching for them. If you do, you may get bad ideas that sound dangerously plausible at the beginning.
The best way is to look for problems, preferably problems you have yourself. Why? Solving your own problems guarantees that 1) the problem really exists (unless you fool yourself big time), and 2) you will have a user from day one — yourself.
“The very best startup ideas tend to have three things in common: they're something the founders themselves want, that they themselves can build, and that few others realize are worth doing. Microsoft, Apple, Yahoo, Google, and Facebook all began this way.”
“The older I get, the more I see how much motivations matter. The Zune was crappy because the people at Microsoft don't really love music or art the way we do. We won because we personally love music. We made the iPod for ourselves, and when you're doing something for yourself, or your best friend or family, you're not going to cheese out. If you don't love something, you're not going to go the extra mile, work the extra weekend, challenge the status quo as much.”
Pay attention to everything that's happening around you. What are you (or your spouse, children or friends) complaining about? Anything not as good as you hope it'd be? What are you buying every week but has lame quality? You may find ideas then!
If you don't have butterflies in your stomach, you are not paying attention.