Shift adds to the selection if another selection is active. To constrain aspect ratio and add to a selection, hold Shift , click, drag and hold, release Shift and hold Shift again. Command subtracts from an active selection. Shift-command selects the intersect of two selections. Option draws the marquee from the center instead of the corner.
Control temporarily activate the magic wand.
Further Information More details on selections and selection tools can be found on the masks section of the page.