This is a bit of a mash-up of ten tool tips and shortcuts, but I find them to be very helpful and wished that I had discovered them when first learning Photoshop!
Selecting Layers — To quickly select layers when working with complex documents – especially those which contain adjustment layers, with the Move tool selected, right-click (or Control click on Mac) in the image over the layer to be selected and choose it from the context sensitive menu. Note: this shortcut works better if if you name your layers. : )
Reposition Selections While Drawing — After starting to draw a selection using the Marquee tool, holding the spacebar (while still holding the mouse down), allows the repositioning of the origin of the selection. Releasing the spacebar (while still holding the mouse down) allows continuation of drawing of the selection. Similarly, when drawing with the Pen tool, if you click to set down an anchor point and need to reposition it, holding the spacebar allows you to reposition that anchor point – as long as you have not released the mouse after clicking to set the point (otherwise you get the Hand tool, as expected).
Cycle Through Nested Tools — Photoshop assigns the same shortcut to multiple, similar, tools. For example, all three Lasso tools have the shortcut “L” assigned to them. To cycle through tools that use the same keyboard shortcut, add the Shit key (for example Shift + L will cycle through the Lasso, Polygonal, and Magnetic Lasso Tools). If you prefer to cycle through tools that have the same shortcut without using the Shift key, choose Preferences > Tools and disable Use Shift Key for Tool Switch. To quickly cycle through nested tools without using a keyboard shortcut, Option -click (Mac) | Alt -click (Win) on the tool in the Toolbar. Or, learn how to create your own custom keyboard shortcut.
Toggling Between Tools — There are several tools which, with the help of a keyboard modifier, can be temporarily toggled to another tool. For example, to temporarily toggle between the Path Selection Tool and the Direct Selection Tool, hold the Command (Mac) | Control (Win) key. Other tools toggle with the help of the Option (Mac) | Alt (Win) key. For example, the Brush Tool will toggle to the Eyedropper Tool and the Pen Tool will toggle to the Convert Point Tool.
Holding the Option (Mac) / Alt (Win) key while dragging the Lasso tool will toggle to the Polygonal Lasso tool – and vice versa – enabling you to combine freeform and straight line segments to your selection. Note: When starting with the Lasso tool, this shortcut might take a few tries to master as you have to be careful to release the Option/Alt key (to return from the Polygonal Lasso tool to the regular Lasso tool) while continuing holding the mouse down. Holding the Option (Mac) / Alt (Win) key while dragging the Magnetic Lasso will toggle to the regular Lasso (enabling you to drag a freeform segment) and the Polygonal Lasso if you click with the cursor (enabling a straight line segment.
Similarly, Option (Mac) | Alt (Win) toggles between the Sharpen/Blur tools or the Dodge/Burn tools – the advantage to using the keyboard modifier with these tools is that many attributes such as brush size remain the same between tools. Option -dragging (Mac) | Alt -dragging (Win) with the Smudge tool will smudge the image using the foreground color (instead of the colors in the image).
And since we’re on the topic of the Dodge, Burn, and Spong tools: with either the Dodge or Burn tool selected:
Option (Mac)/ Alt (Win) + Shift + S targets the Shadows
Option (Mac)/ Alt (Win) + Shift + M targets the Midtones
Option (Mac)/ Alt (Win) + Shift + H targets the Highlights
With the Sponge tool selected:
Option (Mac)/ Alt (Win) + Shift + D will set the Sponge tool to Desaturate
Option (Mac)/ Alt (Win) + Shift + S will set the Sponge tool to Saturate.
The Ruler Tool — To rotate or straighten a layer (without affecting the entire the document), click-drag using the Ruler tool then, click the Straighten Layer button in the Options bar. The Ruler tool can also be used to measure an angle like a protractor. Drag the first line and then Option -click (Mac) | Alt -click (Win) on the either endpoint and drag out the second line. The angle can be viewed in either the Options bar or the Info panel.
Block Mode — The Eraser tool has a special “Block” mode which gives you a eraser in the shape of a square. What’s unique is that when you zoom in and out on the image and use the tool, it erases a certain portion of the screen – regardless of the zoom level.
Erase with History — Holding the Option (Mac) | Alt (Win) with the Eraser tool will erase with history. Note: when Photoshop opens a document, it takes (by default) a snapshot of the document that appears at the top of the History panel – this is the “history” that the Eraser paints with. To change the “History” state (that the Eraser uses to paint from), click in any empty well to the left of the desired state in the History panel. Both states (the one chosen to “erase” with and the one that’s being “erased” upon) must have corresponding layers and be in the same color mode.
Quickly Fill or Erase Areas — The Paint Bucket can fill with the Foreground color or a Pattern (via the drop-down menu in the Options bar). Note: the Fill command (Edit > Fill) also has the pattern option, but the Paint Bucket may be faster than using a dialog box. To erase areas of an image (based on the color clicked upon), set the Paint Bucket’s blend mode to Clear and click in the desired color.
Spring Loaded Cursors — If you want to temporarily access a tool, press and hold the shortcut for the tool. When you release the cursor, Photoshop returns to the previously selected tool. For example, when painting in Photoshop, it can be helpful to use the Rotate View tool to rotate a document on the screen (allowing more natural hand positioning and movement) without actually rotating the contents of the document. With the Brush tool selected, instead of tapping “R” to select the Rotate View tool, press and hold the “R” key to temporarily access the tool. Drag to rotate the document as needed and when finished, release they “R” key to return to the Brush tool. Note: to reset the Rotate View tool, tap the escape key or double click on the tool.
The Tool Bar — The Toolbar can be displayed as either a single or double column (the double column can be very useful if you like to show a large number of tools). Click the double arrows (chevrons) at the top of the tool bar to toggle the layout. This video The Start and Recent Files Workspaces and Customizable Toolbar in Photoshop, demonstrates how to show, hide, and rearrange and regroup tools using the Customize Toolbar editor.
And a Bonus Tip — If a tool isn’t working the way that you think it should, click (or Control -click on Mac) the tool icon in the Options bar (officially called the Tool Preset Picker) and choose between Reset Tool and Reset All Tools to reset the tool options (found in the Options bar) to their default state. This shortcut is a great way to trouble-shoot and reset a tool if perhaps the tool’s blend mode, feather, or other option was changed the last time you used it. Note: this shortcut doesn’t reset the visibility or grouping of the tools, only their options.