Well, potential measures would surely be blackmail, bribery, a small assassination here and there I guess... the Usual, you know.
As for the position of the Sector Governor, that's... quite a tough question. How I imagine it, the Segmentum Branch of the Admanistratum (Lord Segmentum Governor?) would decide who gets the place at the top of the sector. Of course, the Segmentum Governor does not know much about the actual sector, probably mostly about "how productive is it" and "are there any major conflicts threatening the Segmentum as a whole".
Thus, he'll have to get the advice from a council of people (probably AdMech, Ministrorum, Battlefleet, Noble Houses, Merchant Houses, Inquisitors, you name it) who will tell him "Well, that guy is useless" or "That guy is awesome and will double the tithe!".
Other then that, tradition likely also plays a big role, like who owes whom a favor, things like that.
Of course, this picture might be totally wrong, but considering the feudal nature of the Imperium that is my view on how things work... and it'd leave you lots of room for backstabbing, double-dealing and a general atmosphere of "No really, who can we trust to betray us and who can we trust to maybe betray us?"
Edit: Of course you have to keep in mind those organisations NOT directly affiliated with the Adeptus Terra (Adeptus Astartes, Adeptus Mechanicus) thus having their own Agenda.
Edited by darkforce, 19 April 2014 - 10:10 AM.