An employee's behavior depends on a mixture of internal and external factors, some of which are more prominent than others. Internal factors are those within the company's direct control, such as policies, workflows and office culture. External factors, meanwhile, are the ones that aren't directly in the company's control, like the economy and your employees' personal life circumstances.
Compensation and Advancement
Perhaps the clearest internal motivator on an employee's behavior is whether she feels the company pays her what her work is worth and provides her with the opportunities for career advancement that she has earned. That's because income and career prospects are vital to a person's livelihood, and so can profoundly motivate behavior. If the company falls short on either pay or promotion, an employee may begin to suspect the company is taking advantage of her -- especially if she sees her colleagues getting better treatment. You can stop this from taking root by making it clear to an employee early on exactly how she can demonstrate her worth, and then giving her the opportunity to do so.
Workplace culture is prone to cliques and office politics that divert people's energy away from the mission. Discourage selfish opportunism and destructive rivalry by making them liabilities, through discipline and disincentives. In their place promote cooperation, constructive rivalry and respect for people's individuality. Even though workplace culture is a human factor, it's still an internal one, and you can do a lot to control it.
Personal Life Issues
A professional employee should maintain a strong level of work-life separation, but it's inevitable that external issues will sometimes spill over into the workplace. This isn't necessarily a bad thing. Positive life events, like getting married or having a book published, can be a boon to an employee's work ethic. But he can also suffer on the job if he's dealing with negative events like sickness or debts. People need time and energy to deal with their problems, so, to the extent you're able, try and help out a stressed employee by offering him personal leave, a flexible schedule, a reduced workload or a telecommuting option.
Harassment and Discrimination
Harassment and discrimination are a constant risk. This can be external, resulting from broader social prejudices or the psychological problems of specific employees. It can also be internal, resulting from company policies that favor certain groups at the expense of others. If an employee has to worry about being mistreated, he's not going to live up to his potential -- nor will your company. Make it a top priority to watch for these dynamics and uproot them immediately. Most of all, take your employees seriously when they report harassment.