If your motor is a simple DC 9v motor you can make it go in both directions by purchasing a DPDT switch (double pole, double throw) and wiring it as follows:
The switch will have six terminal on the bottom.
1 2 3
4 5 6
Take your two leads from the plus and minus side of the battery and connect them to the two central terminals on the switch (2 and 5). Then connect wires from one side of your motor to terminals 3 and 4, and wires from the other side of your motor to terminals 1 and 6. That way, when you throw the switch in one direction the polarity will be the opposite of when you throw it in the opposite direction, so one side of the switch will make the motor run forward and the other side will make it run backward.
You can get a DPDT switch from any electronics store (Frye's, Radio Shack, etc.) Just make sure the switch is robust enough for your current needs. If it is just a little hobby electric motor you are running, just about any DPDT switch will do. If it is a big motor, make sure the switch is rated for the current you will be sending through it.