Hey I am building the samething. I am also a member at instructables, so if you want I will email you the full instructable. The way without the 555 is easier. It took me a while to understand exactly what they were saying so I will try my best.
The way I saw is you take your DPDT switch and wire it like this
**broken link removed**
The blue lines are wire.
So how this works:
I assumed you modded your servo to spin continuously. When you apply power to the posistive lead of the servo, it spins forward. When you apply power to the negative lead of the servo, it spins backwards. What the DPDT switch does is select which way the motor spins. When you push the switch on (like pushing to switch to the right in picture) you connect the battery + to the motor + and the battery - to the motor -. The moves the motor forward. The motor will spin the arm out of the case and push the switch back. This moves the switch to the other side. Since the SPST switch is noramlly closed, the current will be reversed (battery - to motor +, and battery + to motor -). The arm will start moving back in until it hits the SPST switch. When it does, the switch will open allowing no current to pass to the motor. That completes the cycle, It will now wait for the switch to be pressed again.
In the picture the right side of the DPDT would be labled off and the left side would be labled on.
I did my best a explaining it, but it is a little hard to explain.
Good Luck
Mike