You need to increase the capacity of the battery, this is the mah number, the C is for the rate of charge and discharge. One point to add is that when you increase battery mah the weight also goes up,so the flight times will not double when you double the battery capacity. In short keep an eye on the weight to mah ratios.
Also, as your capacity goes up, you won't need to spend as much money on high C rated packs... As long as your power demand hasn't increased drastically.
ie. 2500mAh x 20C = 50A where as 5000mAh x 10C = 50A
Don't fall for the 25-40C prices if you don't need it.
Keep that in mind, mmmkay?