Aim Small, Miss Small.

Mel Gibson's character in The Patriot says to his sons about shooting a musket, "Aim small. Miss small." Well, that seems good advice when putting. Pick the smallest target you can such as a blade of grass at the cup or some imperfection on the green, dial in the sight line from the target to your ball and pull the trigger. Keep the mental variables to a minimum and trust your instincts and your smooth stroke.