Go for volume. The more ideas, the better. 'The best way to have good ideas is to have lots of ideas,' advised Nobel prize-winning chemist Linus Pauling.
Don't evaluate as you go along. Turning off your internal filters is one of the trickiest parts of creative thinking but one of the most essential.
Once you're on a roll, fresh ideas flow freely. Write all the ideas down as soon as you think of them. Otherwise, like a dream after you wake, they'll be lost for ever.