3. Store your images in a file format that's fairly well known. Using something like TIFF or JPEG will help future-proof your images. I've received letters from people who can't open images that are only four years old because they stored the files in a proprietary image format no longer supported by any manufacturer.
There are some who would argue that you could use a file format that's supported by a large company across several markets (consumer, semi-pro and professional), like Photoshop's PSD file format. If we're talking 30 years, you still might want to consider saving a TIFF right next to the PSD, just in case.
Increasing File Sizes
Q) I optimized an 11.1 MB photo to e-mail in Photoshop and the resulting size was 256 KB; Photoshop showed both files to have 11.1 MB total pixels. Why didn't the total pixels reduce?
A) Photoshop is showing the uncompressed or pixel-for-pixel representation of the image, regardless of whether the file has been compressed. Here's a simplified example. Let's say you have an original image that's comprised of all white pixels. A compressed file format could define the color of the first pixel as R=255, G=255 and B=255 (for the red, green and blue values of the pixel, with 0 being no intensity and 255 being maximum intensity).
Take that file into your image editor and use the software to compress the file so you can e-mail it. The software could look at your image and say, "Hey! The pixel in the upper-left corner of the image repeats for the rest of the image. Let's just store the RGB values for the first pixel and put a note that says to repeat this pixel for the rest of the image."