There are a lot of posts of difficulties switching boot partitions using the on/off switch method. The following method works for me EVERY time, on WRT1200, WRT1900ACV1, WRT1900ACS and WRT3200ACM.
The trick to the timing is to watch the power LED. During a normal power on the power LED will turn on, then after a bit will begin blinking. You can use a serial connection to see how much margin the procedure below allows (it gives at least a second of timing error for the power-off portion). Start with the power switch off, then switch on. Watch the power LED:
Power LED: on (a few seconds)
Power LED: off (a second or two)
Power LED: on (immediately when the power light turns on, flip the power switch off)
That is 1 cycle of the 3 required to revert to the other partition. Repeat the above procedure two more times, making sure to flip the switch off as soon as the power LED comes on the second time.
The fourth time you turn the power switch on you can let the device boot and it should boot into the other partition.
Please add to the wiki!