A precise way to center your marquee tool selections.
Make two guides: vertical and horizontal
Drag out guides from left and top rulers, they should snap to the center when you get close.
(OR: View>New Guides with 50% in the position field)
Make sure Snap to Guides is turned on.
Start your selection close to the axis and hold down the Alt key. The center of the selection will snap to the center axis of the image. Add the shift key to constrain equal dimensions.
This will work with elliptical as well as rectangular marquees

forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=23805400