You can’t always frame your photos as you’d want, but so long as you have some pixels to spare, you can recompose them later. This lets you effectively move key subjects within the overall frame without having to clone them and airbrush out the original.
To pull up the crop tool, tap R in Photoshop or C in Lightroom and Aperture. If you’re working in iPhoto, click Edit followed by Crop. This overlays the image with a frame. Drag the corners to isolate just the part you want to retain, then hit ® in Lightroom or Photoshop, and click Done in iPhoto or Apply in Aperture.
You can constrain the proportions by selecting from the menu in the iPhoto and Lightroom sidebars, or by using the boxes in the floating Dialog in Aperture or the info bar on the top of Photoshop. Doing this lets you choose an aspect ratio that matches your intended output, such as a 6x4-inch print, rather than the camera’s sensor.
Use the Overlay menu on the Photoshop toolbar to reveal a rule of thirds grid, which iPhoto displays by default as soon as you start changing the crop dimensions. It’s always in view when using the Crop tool in Lightroom. You can call it up in Aperture by checking the box marked ‘Show guides’ in the floating dialog.