It's very handy if your initial selection area is off by a few pixels just hold down the space bar to reposition it before releasing the mouse button to snap a screenshot.
Press and hold the space bar (after dragging to highlight an area but before releasing the mouse button or trackpad): This locks in the shape and size of the selection area but lets you reposition it on the screen. A screenshot captured by this method features a white border around the window with a bit of a drop shadow. Click on your desired window to take a screenshot of it. Press and release the space bar: The crosshair turns into a little camera icon, which you can move over any open window. You have a number of other options after hitting Shift-Command-4: Release the mouse button or trackpad to take the shot.
Use this keyboard combo to turn your cursor into a crosshair, which you can drag to select a portion of your screen to capture. This keyboard shortcut captures a screenshot of your entire screen.
