It depends on the battery's size/capacity. Look to see on the label if there is a number followed by the letters MAH. That is the capacity. If there something there like 1500 or 2400, then the obvious answer is to get a new battery with a higher capacity. The latest RC batteries are up over 3000Mah, some over 4000.
The other answer could lie in how the battery is being charged. If you charge at a higher amp rate, you getter better "punch" (initial response), but less longevity. Try charging at a lower amp rate. It may take a little longer to charge fully, but could get you a couple extra minutes of runtime.