The two most important techniques in image editing are, in many respects, the simplest to accomplish:
Look at every pixel
Try to get in the habit of returning to 100-percent (Actual Pixels) view frequently, so you can get a sense of what's going on in your image. People often zoom in closer than this, thinking "the closer the better." Not so. Sure, you can see the pixels, but you're not really seeing the image. (Zen koan or sage advice? You be the judge.)
If you can't fit the image on your screen, start at the upper-left corner (press the Home key) and use the Page Down key to move down until you reach the bottom. Scroll once to the right (press Command-Page Down), and start over. We can't overstress the importance of this procedure.
If you like working zoomed in or out and can't be bothered with getting back to 100-percent view, check out "Tip: Use New Window" in Chapter 2, Essential Photoshop Tips and Tricks.
Build base camps
Our friend and colleague Greg Vander Houwen (you've probably read about him elsewhere in this tome) turned us on to the mountaineering phrase "base camp." The concept is simple: while you're working on an image, don't just save every now and again; instead, create an environment that you can return to at any time. That means taking snapshots in the History palette orbetter yetusing Save As at strategic moments in your image manipulation. It also means saving your curves before applying them, and sometimes even writing down the various settings you use in dialog boxes (like Unsharp Mask).
When you've built a solid base camp, you can always return to it, get your bearings, and start up the hill again. As Greg noted, "I might build a few base camps along the way, depending on how high the mountain is."