Avoid erasing images on the camera. Unless you KNOW the image is a dud, the small screen does not provide adequate detail to assess the image.
Make a backup copy of your images onto a separate drive or storage media from time to time. Especially backup images that are on your camera. A local photography shop can provide a "backup to CD" service when you are traveling or you can email important images to yourself.
Never tamper with an original image, if you must tamper, use a copy.
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