In order to boost an employees performance, know exactly what you want. Doing a better job and working harder are too general. You need to specify what you want done differently. Until you do that, you are not discussing performance.
Think action, not attitude. Yes, attitude is important, but thats up to them. You are not responsible for what they think or feel. You are responsible for what they do. Concentrate on that.
Communicate your expectations. This does not mean tell them what to do. It means explain the results you need and why. Get the employees agreement to produce these results.
Inspect what you expect. Give the employee a chance to perform, and monitor the results. Pay particular attention to the areas you expect improvement, but dont ignore other aspects of the employees work. You dont want improvement in one area to be made at the expense of poor performance elsewhere.
Praise the progress. Compliment the employee on every change for the better that you notice, no matter how slight. Positive reinforcement works.
Refuse to accept poor performance. If you look for improvement and you honestly cant find it, let the employee know. Express your disappointment and ask why nothing has changed. Discuss (as per the agreement) how the employee will get back on track; then repeat steps four and five.