How can a manager motivate an employee who is not performing well in his job?

Talk to the employees who are not performing well educate them.

Identify the problem of the employee. Solve his genuine problem. Arrange his training if he needs. Assign the job as per his qualifications and capabilities. Finally if he doesn't change his behaviour then proper action be taken against him.

I agree completely agree with Mohammad Salim , that first an analysis will help is it because of lazyness, lack of interest, need more training, personal issues or any other reason this employee is under performing.

If they dont get motivated encourage him/her with rewards, if they perform a little give a pat on back , tell others if there is good improvement so that covers recognisition.

If there is no improvement then take neccessary action or issue a strict warning so that either they improve or find a job where they can excel 

Well it depends on so many factors, I mean some employees can get motivated with bonuses & financial rewards, while others may give more weight to the appreciation, they tend to be much more happier if you just expressed how delighted & impressed you're with their performance & achievements, so the first thing to do is gain full understanding of the team you're managing, and then select the appropriate approach to motivate them.

There are several ways one can be motivated, first of all identify the reasons why this particular staff is demotivated, is it personal or professional. Then use motivational tools accordingly.

 

By using reward, recognition, disciplinery actions or threats.