When using the crop tool in Aperture there is no preview supplied to show what the final image will look like. This can make it difficult to get the adjustment right as you switch in and out of crop mode.

However, since Aperture updates the thumbnail in real-time, you can get a preview by making the thumbnail image visible on the screen at the same time.

If using the regular viewer, make the thumbnail large so that the final image can be seen easily. If using full screen mode, switch the film strip to On mode so that it stays on the screen all the time.

[kirkmc adds: I haven't tested this: I don't have Aperture.]