Sometimes it's hard to come up with one good idea. Especially if you don't write them down. So don't come up with one idea. Come up with a thousand.
Sit down. Spend a minute looking at the paper, and ten minutes just writing stream of consciousness. Every idea is good until proven otherwise. Why waste a thought when you can take it and craft it into something awesome, or analyze it and figure out why it would or wouldn't work.
The bottom line is don't restrict yourself. I have a "startup jar" sitting in my room, with strips of paper sitting next to it. Every time I brainstorm, I pick an idea or two, and put it into my jar for future reference. Just remember, there's no bad ideas.