This is probably a re-post, but since I didn't see it on this thread, here it is again: when you park your bike, make sure your wheel and crank are aligned so that the wheel sensor and cadence sensor are NOT aligned with the pickup on the chainstay. If they are, the transmitter will continue to send, and the battery will go dead very quickly.

Same thing with the HR strap: don't put it on until you're ready to ride, and take it off as soon as you're done with your workout, even if you won't have a chance to change clothes right away. As long as it's on your body, it's sending and using battery.