My mom used this one on my brother and I as kids, which came from her dad: "Better to remain silent and let people assume you're an idiot, than to open your mouth and remove all doubt."
Another one from my mom (from her dad) when my brother or I would complain that life wasn't fair... she'd say: "You're damn right life's not fair, and you should be thankful of that, because you'd have a lot less than you have if it was." Use that one on my own kids now... not that we are wealthy by any stretch, but we are comfortable enough (and many are not).
My dad'd dad was quite a colorful character.... and many of his sayings can't be repeated here. One of my favorites though: "Boy, you're so dumb you could fall into a barrel of tits and come out sucking your thumb!"