Reset Dialog Box

When Photoshop opens most of it's dialogue boxes it offers you the ability to click OK or Cancel to accept or cancel the task. Some of Photoshop's dialogue boxes offer a hidden "Reset" option. Items like Levels, Curves, and Hue & Saturation.
Holding the Alt (mac) Option (PC) key changes the Cancel to a Reset, allowing you to reset the dialogue box without having to cancel, and re-open the box every time you want to start over.

Ten Basic Tips

1. Hitting Tab hides all of your palettes. Hitting Tab again brings them back.
2. Shift+Tab (PC) just Tab (mac) hides all the pallets except your toolbar.
3. Hitting the "D" key resets your color pallets to Black foreground, White Background.
4. The "X" key will toggle your foreground and background colors.
5. Use CMD+backspace (mac) Control delete (PC) to fill your layer or selection with the current background color.
6. Option+Delete (PC) Alt+bacspace (mac) fills your layer or selection with the foreground color.
7. Change your paintbrush circle into a crosshair for precise control by pressing the Caps Lock key.
8. Double-Click the ruler on the top or side of your document brings up the Units & Ruler Dialogue box. (handy if you work in both print and web)
9. To quickly change the blend mode of a layer Shift+Alt+(letter corresponding to mode) ie: Shift+Alt+M is Multiply. Shift+Alt+V is Vivid Light. (On a PC use Option instead of Alt.)
11. Using Shift+ the "+" or "-" will toggle through the various blend modes one at a time.

Color Sampler Tool

Here is another useful feature of the Eyedropper tool.
When you compare several samples of colors with each other you don't need to jot down the numbers on a piece of paper. You can press Shift while clicking with the Eyedropper tool (shortcut: "I"). This opens up an extended Info palette. You can click on your image up to four times. For each selector you will be able to see a corresponding color value in the info palette.

Pick Up The Average Color

I've touched on one of the special features of the Eyedropper tool above. It's easy to remember the tool shortcut for the Eyedropper: it's "i" as "eye".
Sometimes when you have a bitmap image, the individual pixels that make up the color of a surface can be very different from each other, especially if the image is noisy. In these cases when you want to pick a color from the surface, you might end up with a much lighter or darker color than the average surface color.
In this case Press command (mac) control (PC) when clicking the area for your sampling to pick a little square of 3x3 or 5x5 pixels instead of just one particular pixel. This way the picked color will truly reflect the overall feel of the color of the surface.