My friend has a NERF gun, which used six 1.5v C batteries in series (9 volts) to shoot discs across the room. These batteries were pretty much connected straight to the motor. Motors are pretty simple, and doing something like doubling the power to them doubles the speed of the motor (causing the NERF discs to shoot farther).
So, he wanted to wire the batteries up so that he could switch between two modes, the recommended battery voltage, and a sniper mode with double voltage. So, we sat down and searched the internets. We found some examples of serial/parallel devices consuming power, but not for the actual power sources themselves. I haven't done much with circuits for a while, but I was able to come up with this diagram.
This is a really simple circuit, all you need is some wires and a DPDT switch (one physical switch, but with two logical switches, each having two possible output paths). Keep in mind that the Series mode doubles the voltage and halves the endurance, and the Parallel mode does the opposite.