I don't think Oleg's method is particularly low-power -what with a 7474! It would want to be a 74HC74!

On some micro's you can put them into sleep mode with interrupts enabled. The on/off pushbutton connects to the external interrupt. The code is arranged so that when it goes into sleep mode, the external interrupt is enabled. When the user presses the pushbutton, the interrupt fires and the micro comes out of sleep. Debouncing etc is done by software. Other devices can have their Vcc controlled via a fet/transistor off a port pin.

Another method is to have a port pin control a fet/transistor to enable power to the micro. When the pushbutton is pressed this turns the transistor on - starting the micro. The micro holds the power on via a port pin to the transistor. A port pin can then be used to sense the pushbutton input.

Erik dislikes SMS abbreviations , my pet hate is 'wanna'. It sounds so 'slang'. "I wanna slap yo mamma!" or "I wanna bust yo crib 'n' pull out me gat n wetcha, holmes I betcha"