You leave it plugged in all day.
That's ok for some devices that automatically stop pulling precious electricity when they are done charging, but do you know for sure which devices those are?
For years I've recommended the Kill-A-Watt
in its various incarnations. The device is simple... just plug it in to the wall, plug something into it and you have instant information on how much juice is being sucked up. It's a no brainer.
Information is power, folks, and whether you're just trying to save a few dollars on electricity or if you are trying to figure out what to put in an emergency kit, you'll appreciate knowing how much power you'll need. Some devices are just fine being plugged in all day, and you'll probably be surprised to find that some devices with the energy star label take far more electricity to run than other devices.
There's no doubt that the money you'll spend on something like this will pay itself back quickly. Personally I just use it for entertainment value (yes I'm just that geeky.)