Please read this link in yahoo on the succession planning of top IT companies.
As a careerologist, here are three lessons i would like to draw from these examples:
1. Successor should be an Insider. If you are changing jobs after 35-40, it is difficult to reach the top. This is part of 'core group' theory.
2. Successor should have a complementary skill set to the incumbent, not supplementary. If the incumbent is an 'organisation-driven' person, the successor could be 'customer-driven'. Whatever you are doing well, please stick to it. Do not try to be like the 'top boss'?
3. It is important to learn the 'soft organisational keys' to drive big agendas in the organisation, not the hard skill set. One can 'learn' these keys only after staying in an organisation for a long time. ( There is an art in learning these keys. If you have it, you are lucky. Others have to consciously develop this art.!)