Terry Pratchett has a good metaphor for a good way of thinking in his Tiffany Aching books. Second, third, etc thoughts. Basically the idea that you shouldn't just trust whatever your thoughts say, you have your second thoughts monitoring them. And then you have your third thoughts monitoring that. I've always found it extremely helpful to pay attention to what I'm thinking; many times I've noticed a thought slipping past that is very obviously wrong and picked myseld up on it. A few times I've even agreed with the original thought upon further analysis, but analysis that it definitely needed.

I'm not sure the second thoughts metaphor is the best way for explaining it to people mind you. Perhaps something such as "Pay attention to what you're thinking, you'll be surprised how often you disagree with your own first thought."