I have this checklist and "3-2-3" is the way I keep it in my head (you can also tape "3-2-3" to the backside of your camera if you need):

3 steps before going out with a camera:

These are the most critical checks. You rarely are going to have a chance to recover from not having checked them once you're out with the camera.

• Battery inserted and with charge
• Memory card inserted and with space available
• Quick power on test

If you happen to have a second battery or card, try to always carry them too if you are leaving for more than a hour. If leaving for a full day, take the battery charger too if possible.

2 steps during a session, before turning off the camera:

There are many other things to check, sure, but the two items bellow can really ruin your next shot if not checked.

• Turn on Image Stabilization (critical if your next shot is handheld but the previous one was not)
• Set white balance to auto (critical if you are changing light conditions, a quite common situation while traveling or indoors)

If you manage to remember it after the above checks, it never hurts to reset ISO too. On the other hand, a higher than needed ISO setting while not a good thing, probably won't ruin your next shot like missing one of the two checks above.

Everything else is usually quickly noticeable before your next shot (due to their direct interference with exposure levels).

3 steps after the session itself:

• Backup the card(s), even if not erasing them yet, wherever you can
• Charge the battery (and replace it with the spare if available in order to rotate them)
• Charge the spare battery, if the case

A long answer, I know, just keep the "3-2-3" in mind and you'll see that it's not that much trouble. :o)