Select a Layer Outline
Command-Click a layer thumbnail in the Layers palette to load a selection of its outline, including any anti-aliased (non-jaggy) edges. If your layer has an inherent opacity set (for example, if you opened a partially transparent PNG image), the opacity difference is included in the selection. Normal opacities set in the Layers palette, however, do not affect the selection.
Use Multiple Selections
Once a selection is made, there are a number of operations you can perform to modify your selection. Holding Shift and making an additional selection will add to your current selection, and Option will remove from it. Holding both Shift and Option will do something unique: wherever your two selections intersect will become the new selection.
Combine Layer Outlines
Using the first Command-click thumbnail hint with Shift, Option, or Shift+Option modifier keys, you can add, remove, or intersect selections using outlines of other layers. Your cursor will show +, -, or x to indicate which type of operation will be performed, respectively.
Move Selection While Dragging
While in the middle of dragging a selection, you can hold down the spacebar to move around the origin of the selection. This is extremely useful if you find that your selection is a bit off — Instead of re-making a new selection from scratch, you can make adjustments “on the fly.” When the spacebar is released, the selection seamlessly drops back into the default “grow” mode, using the new origin as the starting point.
Start at the Center
Hold down Option after starting a selection to expand from the middle, causing the outline to grow symmetrically in each cardinal direction. Add the Shift key into the mix Shift to maintain a square shaped ratio.
Fixed Ratios and Sizes
Using the selection tool options, you can set a fixed ratio or specific size, both of which are great for slicing out content with a pre-determined size, like that of a computer wallpaper. Using the ratio, you can select a portion of an image that would fit on your desktop. Once you have the portion selected, you can scale it down to the native resolution of your display, being sure that it will scale proportionally to the correct size.
Quick Mask Mode
By flipping into Quick Mask Mode, you can use paintbrush tools to “paint” the beginnings of a selection. When you leave Quick Mask Mode, the painted area becomes a selection which you can use right away or modify further.
Use the Select->Transform Selection command to distort the current selection. Note that this applies to just the selection outline, not the content within it. (To change the content, do Edit->Transform->pick a type).
Selection Paste Target
By making a selection and then pasting content into it, you can target exactly where the pasted content will land. Without a selection, pasted content simply gets dumped in the middle of the document.
Save Selections with the PSD File
Once you have your beautiful selection made and ready to use, you might consider saving for future use if it was particularly complicated to make, or if it’s a handy, reusable shape. Photoshop provides two simple commands for saving and loading selections. Choose Select->Save Selection to commit your selection to a given name. Retrieving it is as simple as picking the Select->Load Selection command, and choosing the name you saved the selection under earlier. Best of all, these named selections are included in the file, meaning you can save and re-open the document, and reload selections at a later date.