In his book "The Way of Baseball" Shawn Green describes what turned around his career. it was hitting off a tee so frequently that he no longer had to think about his swing. He'd do it before games and describes getting into a zen-like state.
When my son was younger and in a slump I tried to give him a reason to believe that the next game would be different...using a different bat, a slight adjustment...anything to make it seem like a fresh start. These days if he has a rough day he goes back to watch video of successful at bats in the past (like someone above mentioned about Winfield).