How to Use the Photoshop Marquee Tool
From Eric Miller, former About.com Guide
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More Selection Options
With the “rectangular” and “elliptical” marquee tool, you can hold down the “shift” key while dragging the selection to create a perfect square or circle. Notice you can still change the size, but the proportion remains the same. Another useful trick is to move the entire selection as you create it. Often, you will find your marquee starting point isn’t at the exact intended spot on the canvas. To move the selection, hold down the space bar and drag the mouse; the selection will move instead of resizing. To continue to resize, release the space bar.