Have you ever tried to crop something with the Selection Marquee only to realize that your initial placement of marquee was off by a few pixels? Arghh!!! That can be frustrating, forcing you to redo the entire selection. But that's not how the pros do it. Instead, while you drag the selection out hold down SPACE with your thumb. Viola! Now you can nudge the marquee around the screen without yet committing to its final dimensions.