Quantity: generate lots and lots and lots of ideas!
No judging: if you judge each idea as it comes up, you stop the flow. Instead, have the brainstorming and the evaluation phases be separate.
Write everything down: if you write down some ideas but not others, you're already judging.
Build on other people's ideas: there's nothing totally new and sometimes just a little change in an existing idea is a breakthrough.
If you'd like to have a poster of these guidelines, here it is: