Train, develop, and coach employees. After you have established minimum performance standards, give employees the help they need to meet or exceed those standards. If a salesperson is unable to meet quotas, you might send that person to a course in lead generation. Accordingly, if a hitter is batting .097 he may need more time in the batting cage or to review film of his swing with the hitting coach. Be patient - to a point.
Nobody is perfect.
We all make mistakes. A batter might have an 0 for 22 slump. However, it is not fair to the team, or to the non-performing employee themselves, to allow it to go on forever. He ll come around in time is not an appropriate answer when you know it has dragged on too long already. As a manager, you get paid to make the tough decisions.