Not checking cards and batteries
How often have you found yourself out shooting only to discover that you only have room for a couple of pictures on the card in the camera? Having to delete images to free up space while out shooting is no fun, and takes time! Try to get into the habit of downloading the contents of your memory cards, and formatting them, every time you return home from a shoot. Ensure your battery has enough juice to get you through a shoot, too. Excessive use of Live View, playback mode and shooting in cold conditions can all reduce the shooting time considerably, and it certainly makes sense to take a back-up battery in these circumstances.