Shucks. I was all warmed up to say amulet, but Kilgore Trout beat me to it. Good going Kilgore. Perhaps it is too religiously keyed, or taken over by the digital world, but icon also fills the bill, small figures or paintings that serve to focus on a belief. Good luck bracelets and necklaces and rabbits feet have also served this purpose. For a while, major league baseball teams seemed to all wear a specific kind of necklace that brought good luck. Then there are always the good luck key fob, t-shirt, pocket knife, belt, etc.
A charm. How about that? Or mentally, a mantra.