set a fixed dimension in the crop tool. In Photoshop, you can tell the crop tool that you want, for example, an 8x10 at 300ppi. You can then 'draw' the 8x10 selection. Photoshop will maintain the proportions, and convert the final crop to the desired size.
How would I maintain (lock) the "aspect ratio" for cropping photos in Photoshop Elements V 3.0 ?