On the other hand, if you give an assignment that seems just beyond an employee's reach, people usually will rise to the occasion. People occasionally fall short of their best, and when that happens they disappoint themselves and you. But people almost never fail intentionally. Give them the benefit of the doubt. Put work in context.
People perform best when they feel they are part of something.
Help people understand how they contribute to the overall mission and goals of the company. Employees who don't see that connection will wonder why it's important to do a good job. When making those assignments, be clear. One boss routinely made assignments by scrawling instructions on scratch paper and leaving the scrawls on an employee's chair or in his in-box. Inevitably, employees would take the scrawl to a co-worker for help in deciphering it.