A little while ago, Noel Carboni started a thread on the Rob Galbraith Forums entitled "Good Habits". I thought this was a pretty good idea, so I've pinched it. 
Some of the points are touched on in my article, I've got the camera, now what?, but others are new. If you've a good tip you think should be added, please e-mail me.
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Check your camera settings before you start a shoot. I have preferred settings, so I check ISO, exposure compensation, metering mode, focus point, flash compensation, file format, custom settings (mirror lock-up). Some of these, you may have changed last time out on purpose, others, like focus point, can easily be changed by mistake.
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Check the lens settings: AF or manual, image stabilisation and focusing distance (if your lens has switches for these).
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Keep your batteries fully charged - you can't hurt them by keeping them topped up - at least, not the Li-ion ones in the D30/60/10D. If you have more than one set of batteries, rotate them to spread usage.
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If you are going to be using an external flash, are the batteries charged? Do you have a spare set? Are the settings set to your preferred defaults?
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Is there a CF card in the camera? Have you downloaded all the images from the last shoot? Do you have extra CF cards, if necessary?
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Keep a jumbo blower handy for when you take the lens cap off. Don't use the brush on the lens, use it on the inside of the cap: dust can collect there and transfer itself to the lens when you replace the cap.
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If you're carrying the camera, use a strap - round the neck or wrap it around your wrist.
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If you're using a tripod, hang on to the camera while you move it. If you're moving some distance, don't hang the camera end over your shoulder; turn the tripod upside down to dangle the legs over your shoulder instead - making sure not to poke anyone's eye out.
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When you're changing lenses, take care not to drop anything and try to keep dust from getting into the camera and on to the sensor.