Use the Marque toolto select the exact are you want, then copy, new file, paste.

When you get overages just trim them using the Magic Wand.

If you are off your mark, zoom in wheere you want the corner of the MArque to sit, grab the upper left hand corner of the frame, just off the picture (up inside the ruler) drag the cross hairs to your mark and zero should be awful close to here you want your mark to be.

If not, then go to Image > Image Size, and increase the size of your picture, then go through the step for setting where ZERO sets inside the frame.

I don't have time right now to do a mini-tut. I hope my babbling helps...

Yank My Doodle, It's a Dandy!