Been playing with P'shop for a good 11yrs, and still not too great with it. I have a buddy who's a designer for People magazine, and these 3 basic general things he taught me are probably the most important things I've learned, thought I'd share:
1) If you're going to crop or resize an image, do it first before applying filters or making adjustments. Many of these functions take into account the total sum values of an image, and you may be affecting the final result by allowing it to calculate parts you're going to discard. Especially true for highly contrasting areas.
2) Always adjust levels first before anything else (Image>Adjust>Levels..). The outer arrows (1 & 3) should be at the beginning and end of the graph. In this case, #1 should be moved to the right a little bit. Then you can adjust #2 to your liking.
3) Don't overuse the Sharpen filter. What looks good on your monitor may not look as nice on someone else's. If you don't really need it, don't use it. It can cause graininess, pixelation, or jagged edges (known in the gaming community as "the jaggies").