Mastering PhotoShop seems to take a lifetime. Not that I would put myself in that esteemed camp. I came across a few surprisingly easy but effective tutorials at the Photoshop Café here and here.
If you are a keen photographer, it’s hard to pass up this gem of a book The Photoshop CS Book for Digital Photographers by Scott Kelby. If you can find a better hands-on book, buy it and please let me know what it is! Here’s just one very simple tip of many more involved examples from the book: to display your work press ‘f’, ‘f’ and then ‘Tab’. The first ‘f’ centers your work; the second hides the menu bar and displays a black background. ‘Tab’ hides the toolbars, Options bar and palettes. Just press ‘f’ and ‘Tab’ to return to normal working mode.