Turn it on when I take it out the bag and just leave it on until it goes back in again.
It sometimes takes the odd shot with the vertical grip over my shoulder but as Rob said above, if I turned it off I'd forget when I needed it quickly.
The power switch is a PITA to use on the 1D, better design on the 5D and even better on the xxxd series. The battery is more than capable, 4 days shooting time on snooze 1500+ shots and still ~40% left.