I like short, Primal one-liners - they help me make better decisions on the fly, and it is easier to remember stuff when there is a catchy sentence involved. An MDA member posted a good one on another thread that I really like - "Don't eat anywhere that has more than three locations." Mark's 10 Rules are classic.

Of course one should do the in-depth reading and research behind them, but still.

So - what are your personal favorites, however they have helped you, even if they are "oldies but goodies"?