When you need to control a DC motor (such as a DC linear actuator) you usually need to be able to swap the polarity on the wires going to the motor. 

A double pole, double throw switch is used for this purpose but you have to wire it up correctly to reverse the polarity going to the linear actuator. 

Step 1: Required Materials

You need the following materials: 

-DPDT rocker switch 
-insulated disconnect terminals (female) x 6 
-wire 
-wire cutter/crimper/stripper tool 
-a linear actuator or DC motor to control

Step 2: Connecting the Wires to the Disconnect Terminals

First you need to cut a bit off each wire that will be going to the motor. This is the jumper that will allow the polarity to swap.

I'm using blue and brown wire. THE COLOR OF THE WIRE IS NOT SIGNIFICANT. You can use red and black, or any combination of wire color. Because you'll be swapping the polarity back and forth on the actuator, the color of the wire, and whether they are on the right side or the left doesn't matter. 

Twist together one end of the blue jumper wire and the end of the main blue wire. Then push this into the disconnect terminal and crimp with the wire tool.

Do the same for the brown wire.

At the end of the blue jumper attach another disconnect terminal.

Do the same for the brown wire.

Attach the main blue terminal to one of the poles on the side, attach the main brown termainl to the pole right next to it.

Attach the blue jumper connector to the opposite corner, and the brown jumper to the other corner to form an 'X' across the switch.

Step 3: Connecting the Wires to the Disconnect Terminals (for Power)

Similar to the last step take another wire that will go to the +12V DC (or whatever you are using). I am using blue wire. 

Attach this to a disconnect terminal. 

Do the same with another piece of wire (I used brown). This will go to ground. 

Attach these to the center poles of the switch. 

With my switch it doesn't matter which poles it's attached to, as there is no up and down for the switch. If you don't like it just turn the switch upside down. Otherwise test before installing the switch.

Step 4: Done!

Done! Now we can connect the power to the switch and wire the DC motor to the switch so it can reverse direction.

I use these for linear actuators (http://www.FirgelliAuto.com/), but you can just as easily use these for any DC motor where you need to reverse the polarity to change the direction of the motor.

Cheers!

Step 5: UPDATE: Wiring Diagram

I've had a number of people ask me about a wiring diagram, as the photos may not be enough on their own.