Theres more to copying and pasting than meets the eye. Make your life easier by learning the tricks behind smart copying and pasting.

Bypass Pesky Layers With Edit>Copy Merged
When you choose Edit>Copy, or type Command-C (Mac), or Ctrl-C (Win), Photoshop will copy the currently selected area of the active layer. If you'd rather copy the visual look of your image (regardless of how many layers it's made from), then either choose Edit>Copy Merged or add the Shift key to the normal copy keyboard command. If you don't want to copy all the layers, just turn off the eyeball icon next to the layers you don't want included.

Control Where You Paste
When pasting something that you've copied, Photoshop will center the result within the current view of your image. If you want to control where the image is pasted, then make a selection first and Photoshop will center the pasted image on the current selection. If you want to paste something into the corner of an image, make a tiny selection that touches the corner and Photoshop will attempt to center the pasted information on that selection, but it will not let any part of it extend beyond the edge of the document.

Limit Where The Pasted Information Shows Up
If you'd like to limit where the pasted information shows up, make a selection before pasting the image and then choose Edit>Paste Into or type Shift-Command-V (Mac), or Shift-Ctrl-V (Win). That will crop the pasted information into the selection that was active by adding a Layer Mask to the layer that contains the pasted information. After you've done that, you can use the Move tool to reposition the pasted info within the shape of that selection.

Know What You're Copying
If you copy and paste between two documents, Photoshop keeps what you've copied on the clipboard. If, on the other hand, you drag with the Move tool between those two documents, then the clipboard remains empty, which can save memory and scratch disk space. If you drag from the Layers palette, Photoshop will copy the entire layer regardless of any selection you might have active at the time. If you drag from the main image window, your copied information is determined by where you click. Click within an active selection and only the selected area of the active layer will be copied. If you drag outside of the selection, then the entire layer will be copied.

The Shift Key Makes a Difference
If both documents are exactly the same size (as determined by their width and height in pixels), then holding Shift when dragging between documents will keep the information in the same position in both documents. If the documents are of different sizes, then holding Shift will cause the information to be centered in the destination document. If a selection is active in the destination document, then holding Shift will center the dragged information on the active selection.

Keep Your Clipboard Clean
If you ever copy/paste a huge portion of an image, then you might want to choose Edit>Purge>Clipboard after you're done pasting the image. That way Photoshop won't have to keep the behemoth of an image on the clipboard until you decide to copy something else. It's kind of like clearing the palate between courses, and it helps Photoshop run more efficiently.

Save Memory With Export Clipboard Option Turned Off
If you don't plan on copying something from Photoshop and pasting it into a different program (like Adobe Illustrator), then you might want to consider going to your General Preferences (under the Photoshop, Edit, or File menu depending on your OS) and turning off the Export Clipboard option. With that option turned off, you will not be able to copy/paste between Photoshop and another program (although you can still go the other direction), which can save a bunch of time and memory when switching between programs.

Pasting From Illustrator
When Adobe released Adobe Illustrator 10, they started to use PDF as the clipboard format. That caused a change in how Photoshop dealt with information pasted from Illustrator. If you miss the ability to paste Illustrator information as paths or shape layers, then try turning on the AICB setting (it stands for Adobe Illustrator ClipBoard) in Illustrator's Files & Clipboard preferences. That will make Illustrator 10 act like previous versions and bring back all the options that were available when pasting into Photoshop, which is much more versatile.