Dear Siva,
When you promote your staffs, keep it mind that alike recruitment, it should be competency based promotion. For this, first identify the competencies of every single position.
Many times promotion is given based on past performance. However, past performance is no guarantee for handling higher assignments. I wish that you should not fall in this trap.
I remember long time ago what my Executive Director use to say - a very good worker need not be good supervisor. Assess that person's qualities vis-a-vis requirements of that job. Else wrong promotion affects the company two way. One is you lost good worker and second is that you created inefficient supervisor.
However, this is old situation. For this it is better to create a career plan for every employee. Promotion should be the part of career planning of that employee.
Secondly, employee training plays here major role. Groom your employees so that they can handle higher assignments effectively.
Dinesh V Divekar