Take a number of shots using lines to create a sense of depth.
The images above demonstrate how lines can be used to give a sense of depth. Lines that are parallel give us the impression that they are meeting in the distance and our eye is naturally drawn to this point.
Take a number of shots using lines to flatten the pictorial space. To avoid the effects of perspective, the sensor/film plane should be parallel to the subject and you may like to try a high viewpoint (i.e. looking down).
These images using lines to completely different effect. The lines do not draw the eye into the picture but rather, one is left scanning the surface for a focal point. The feel is much more abstract.
This exercise demonstrates how understanding lines and the illusions they create is a very important skill for the photographer, to add a sense of depth or flatness to their images.
Cropping and Framing
I understand framing to be how the elements of a picture are positioned within the frame as the photograph is being taken, whereas cropping is something that would be done after the picture is taken, to remove any unwanted elements from around the edges of the frame.