The one I remembered today was "Ask women about your idea". His reasoning was that most men always thought in competitive terms, i.e. "I want to crush the competition", "We have to win" etc.
Now I'm not a fan of stereotyping. I think its simplistic, lazy and leads to errors, but as a movie character once said, "I'm like my grandmother, I stereotype. It's faster." :-)
Many men think in aggressive terms and a lot of women don't - which means they're often better (on average) at evaluating ideas. Guy's advice was that asking women about your idea (if you were a man) was a good way to make sure you weren't doing something for purely competitive reasons.
This is a long-winded way of saying - if you're building products, think hard about the competitive landscape. Its key to a good strategy. But to win, you must always focus on the user and what you can do for them. If you're not sure that you are (and you're a guy), make sure you've asked a woman for an opinion - its likely to help you get to a better decision.