My favorite is because I need some help remembering the difference between coral snakes and king snakes: Red and yellow kill a fellow, Red and black is safe for Jack. (The snakes have stripes. Coral snakes are venomous and the red stripes touch the yellow stripes.)
Also, "stay off the ice until after Christmas" is useful where I live because the ice on lakes may or may not be reliable until then. After that, there's usually been enough cold weather to make the ice safe to walk on (but you really need to use your head). This only has local validity, though.
Then there's the ever popular "Get off the water when you see lightning". Always good advice.
Got a favorite that's useful and true?