There is little a developer can do to change the fill rate that I have figured out. I have noticed the rate climb as more countries are added to the iAD program. Obviously the mix of countries where your app is popular can make a difference since there is no iAD program in some countries.
Beyond that there is also the issue of available ads. If nobody is advertising then the fill rate will fall.
What seems a mystery to me is why I have a large disparity in the ePM. I have one app at $2.37 and another at 88¢