One should have good relations with Govt(Politicians) to reach at these posts. For example a PSP can be promoted to DIG on merit. But IG is a political appointment.

Masood Shareef Khattak was Major in army and he was appointed as DG IB. Once he said in an interview: " when i was major i used to inform PM Benazir Bhutto about possible dangers and she trusted me and appointed me DG IB"(in this case you can imagine his influence---an army major's good relations with Prime Minister)

or current DG IB Akhtar Gorchani served as CCPO Karachi. And CCPO is also a political appointment.

So one should develop good relations with politicians to reach at such posts. I.e. Ambassdor, IG, DG FIA, DG IB, chairman NAB, CCPO,Secratary or Chief Secratary etc.