No one is born with brilliant ideas. A professor of mine once said, "If you don't think you have any good ideas, lower your threshold for what you consider good."
Take any idea (even a bad one), and plan out how you'd implement it. Rinse and repeat over time. Eventually you'll start getting better ideas, and seeing opportunities where you see problems.
Seconded. Obviously you don't want to work on crap, but keep in mind that Google was supposed by many to fail based on the existing maturity of the search market:
http://www.bvp.com/Portfolio/AntiPortfolio.aspx
Some of the ideas you've tossed aside were wins.
For that matter, just start writing down every idea you have, whether you think it's good or not. Keep track of your batting average. Keep track of the kinds of things you were right on. Obviously a long-term trending thing, but probably useful at some point in your future.
From my experience writing every single new ideas down really helped have more ideas and better ones too...
I feel that just the act of writing them down forces you to think more about finding great ideas...
Um, lowering your threshold for what you'll let yourself think, is one thing. If anything, I would suggest raising your threshold for what you consider good enough to implement.
My advice would be to find a creative cofounder instead of relying on yourself to come up with the idea. Straining your mind too hard will probably just produce something me-too that won't really work.
Take any idea (even a bad one), and plan out how you'd implement it. Rinse and repeat over time. Eventually you'll start getting better ideas, and seeing opportunities where you see problems.