Sometimes this requires tact. For example, if you are unable to help someone by doing what they ask, you can deflect their unhappiness by helping them find someone else to take your place.
Use the following guidelines;
Be sincere. Don’t promise anything you can’t or won’t deliver.
Know exactly what it is you want the other person to do.
Be empathetic. Look at it from the other person’s point of view.
Consider the actual benefits the other person will receive.
Match those benefits to the other person’s wants.
Couch your requests in terminology that vividly portrays the actual benefits the other person will receive.