I like the series very much, but these are the kind of things that fall in the "just accept it" category. I don't remember at any point feeling that Martin has portrayed any actual ruling very well, especially not good ruling.
A good, although not a highly prominent, example of this is Tyrion's stint running the drains of Casterly Rock. He apparently did a brilliant job, but no details were given what he actually did. Another example is Littlefinger. He's a wizard with money, we're told many times, but if I had to give a single example of this wizardry, apparently he has borrowed a lot of money? I don't know anything else he has actually done. Tywin's just the same. As the above poster said, he used his endless resources to restore his family to its former glory. He did this by, uh, being ruthless?
Basically, the characters that are actually good at politics are good because we're told they are.
Tangential, but what are the odds Tyrion will at some point lead an attack on Casterly Rock through the drains? Quite high, I'd say.
I've always assumed that Dany will eventually sail the long way around the world from Asshai and land somewhere around Casterly Rock, and that Tyrion's drainage pipe assault would be her first conquest on Westerosi soil.