If you plan to use the history brush, be sure to save and reopen the file if you crop the image or change its color mode. Otherwise, when you try to use the history brush, you'll get a message indicating that it can't be used because the current state does not match that of the history state (canvas size or color mode).
Under preferences, Painting Tool cursors can be set as "Standard," to display the pointers as tool icons; "Brush Size" to display the cursors as brush shapes indicating the size of the currently selected brush; or Precise, to display the pointers as cross-hairs. If you've checked Standard or Brush Size, you can access "Precise" cross-hairs on the fly by simply pressing the Caps-Lock key. Release the Caps-Lock key to return to your previous display setting.
To zoom in when using any tool, just press Cmd-Spacebar (PC: Ctrl-Spacebar) and click the mouse button to toggle to the magnification tool and zoom in. To zoom out, press Option-Spacebar (PC: Alt-Spacebar) and click the mouse button. To quickly zoom in to an image, press Cmd + (the plus sign). To zoom out, press Cmd - (the minus sign). PC would be Ctrl + and Ctrl - respectively.
Change your mind about cropping an image? Press the Escape (Esc) key to cancel it.
To increase or decrease the brush size while using a brush, press the left bracket [ to decrease or right bracket ] to increase. To increase or decrease the softness of the brush tool, use shift-left bracket [ to decrease or shift-right bracket ] to increase.
If you make a selection with a Lasso tool and want to feather it, just press the Return key. Focus will be in the Feather box (in the Options Palette), and you can quickly enter a value.
You can cycle through nested tools by pressing the quick key for the tool along with the shift key (refer to your user's manual). Example: for the lasso tool, press "L," then to cycle through its nested tools (Polygon Lasso and Magnetic Lasso), press Shift-L. If you'd prefer to just cycle through by pressing the letter only (no shift key), change your General Preferences setting by unchecking "Use Shift Key for Tool Switch."
Do your palettes sometimes get in the way of your work? Press the Tab key to make your palettes disappear (press Tab again to make them visible again). Press Shift-Tab to hide all but the tool palette (Shift-Tab will bring them back).
To change the Cancel button to a Reset button in dialog boxes, hold down the Option (PC: Alt) key.
Incremental Movements
While the move tool is selected, you can press the up/down/left/right arrow keys on your keyboard to move a selection in increments of 1 pixel. To move in increments of 10 pixels, hold the Shift Key while pressing the arrow keys.
Instead of navigating with the mouse to Image from the menu bar, then Image Size or Canvas Size to modify those settings, use this shortcut: Windows users, just right-click on the title bar and choose Image Size or Canvas Size from the contextual menu; Mac users, Ctrl-click on the menu bar.
To hide all layers except for the one you're working on, hold down the Alt (Windows)/ Opt (Mac) key and click the eye icon in the column to the left of the layer you want to remain visible.
In Photoshop 6.0, you can check Auto Select Layer to automatically select the layer of the item you click on. But sometimes that might be inconvenient. An alternative method is to leave Auto Select Layer unchecked and just press the Command key (Windows: Control) while clicking on an object to make that object's layer active.
Shortcut to move selected layers up or down in the layers palette: Ctrl-] (Mac: Cmd-]) moves it up, Ctrl-[ (Mac: Cmd-[) moves the selected layer down.
To copy the pixels of a selection on all visible layers instead of just the active layer, press Ctrl-Shift-C (Mac: Cmd-Shift-C). If you just press Cmd-C or Ctrl-C, only the pixels on the active layer are selected.
To create a composite layer and still retain all your working layers, create a new layer on top, then press the Option key (PC: Alt key) while selecting Merge Visible on the layers palette flyout menu.
To preserve the transparency of a layer, use this shortcut: Alt-Shift-Backspace (PC) or Option-Shift-Delete (Mac).
Did you know you can drag and drop a layer style from the Styles Palette directly to an object on a layer to apply the effect?
It's a good idea to get in the habit of naming layers appropriately. It may be a time saver if you ever have to go back into the file, and if someone follows behind you, they'll be very grateful. Don't forget that layer sets are very useful as well in organizing your layers.
To quickly kern between characters, such as WA, place your text cursor between the characters and press Option(PC: Alt) + right arrow key to add space, or Option(PC: Alt) + left arrow to decrease space.
Before rasterizing a type layer, make a duplicate of it in case you need to edit the type in the future. It's easier than trying to guess sizes and typefaces weeks, months, or even days later. Or as an alternative, especially when trying to keep file sizes as small as possible, is to include that information when you rename the layer.
Need to undo more than the last step you did? Multiple undos are possible with Cmd-Opt-Z (Ctrl-Alt-Z).
To fill a layer or selection with the background color, press Cmd-Delete (Control-Backspace on a PC). To fill a layer or selection with the foreground color, press Option-Delete (PC: Alt-Backspace).
To repeat a filter you've just done, press Cmd-F (Mac) or Ctr-F (Windows).
Are your ruler increments in pixels, but you want inches for your current project? Simply double-click on the ruler to bring up the preferences dialog box, and choose the increments you want (inches, pixels, etc.)
Press the Option key (PC: Alt) when clicking the New Adjustment Layer or New Fill icon to bring up the Adjustment Layers blending modes.
If you're in a dialog box and change your mind about the changes you made, don't click cancel. Instead, revert to the original state by pressing the Option (Mac)/Alt (Windows) key. The "Cancel" button will become a "Reset" button so that you can start over without exiting the dialog box first.
To load a path as a selection, Ctrl-Click (Mac: Cmd-Click) on the path's name in the Paths Palette; using the same commands, you can load a channel as a selection while in the Channel Palette.
Dialog box shortcut: To respond to a "Yes" / "No" choice in a dialog box, you can simply type the letter "Y" or "N" on your keyboard.
If you have a large area to erase, draw a marquee around it and then delete it rather than using the Eraser tool.
Clicking a color with the eyedropper makes it the foreground color; to make it the background color instead, just Opt-Click (Mac) or Alt-Click (PC).
When you want to apply special effects, are some of the filters grayed out? Check what mode you're in...some filters will not work with CMYK images; change your mode to RGB in order to apply the effects
To quickly bring up the General Preferences options (display, file saving, cursor, etc.), press Cmd-K (Ctrl-K in PC).
Is Photoshop suddenly behaving unexpectedly? Your preference files may be damaged. Refer to your User Guide for instructions on how to replace them.
1 Taken from, http://www.photoshopfoolery.com and used here without permission for educational puposes only!