DIY design blog, See That There, has been putting out a great series of printable template designs for iPhone users looking to customize their favorite phone using their unique colorful patterns with a clear phone case...
These custom case design templates were inspired by Craft and Creativity's project (shown below) and designed specifically to fit cases from iLuv and also Incase's Snap Case. Simply print these designs out onto high quality matte or gloss inkjet stock, cut them out using an X-acto blade, and pop them into your clear case.
Craft and Creativity's custom iPhone cover used decorative tape, wrapping paper, and photos to customize.
If you want to plug in your own design, pattern or photo, here's the blank EPS file to use in Illustrator or similar illustration design programs...perhaps using one of these serene desktop wallpaper images if bright & poppy aren't your thing.