I have blatantly copied Crossroads' sleep mode collection on my current project ( except I have a separate interrupt pushbutton to ground ), and I also power my transmitter and 3 cmos counter chips from an output pin ( make sure you switch all inputs to the cmos chips to low before powering them down or the protection diodes will short the input to ground via the vcc being low - I blew one micro that way )
I am using a 9v battery and a micropower 5 volt regulator that is always on , and my unit draws 110 microamps in sleep mode, which with a 580mAh duracell should last about 5000 hours = about 7 months.