I have a circuit which required minimal components, and ran off a 9v battery, so had to achieve absolutely minimal current. My own method was to use a low-power 15v d-type (Fairchild's MM74C74M was what I used - the only problem is that it's about 10x the cost of a normal 'cheap' flipflop - more on this later) with an inverted Q output, connect the inverted Q to the D pin (so that each time the clock is actuated, it inverts), the clock to a pushbutton (debounced by connecting a .01uF cap to each of the leads), then to 5v, and the clock also to a pull-down resistor so that it rests at ground. Next the non-inverted Q was sent to the gate of a mosfet to control power to the rest of the board.

This circuit in my application achieved .004mA off power and powered on/off with great reliability.

A few notes:
The reason I had to use a 15v flipflop was that the only other ones available were 6v or lower, and the battery was 9v - A voltage regulator consumed several mA by itself, and would run down a 9v in a day or two by itself. So, moving the voltage reg to after the on/off is critical. In order to ensure reliability, the 15v was used in production models -- BUT -- I was able to successfully use such 'cheap' pin-compatible models as the 74hc74 (6v), and they work fine. I'm guessing because the only power throughput is to the mosfet, which is miniscule, it probably would not be a problem using a flipflop underrated for the voltage... But like I said, for anything 'leaving the workbench', the 15v will be on there.
Any thoughts?