The nice thing about cropping is that while you're at it, you are really looking closely at what effect different subjects in the image have on each other and how eliminating them can change the balance and feeling of a photo completely.
I took this picture at a fishing village close by my house. I think it's suitable for this exercise since there are so many different elements to the picture. This is what happens if I crop out the pelican.
With the boat pointing at the wave and the other boat in the background the theme of the image has changed. This image is about fishing, and being at sea.
In this crop, I choose to only frame the bird and the boat and leave as much of the sea and sky out. The focus should be on the bird looking at the boat and like this representing the dependency on the boat and the fishing village it represents. The frame is square, which in this case works ok, since the shape of the bird creates a movement to the center and therefor balancing it out, I guess :-)
This picture is taken on a beach south of Dakar. I thought it would be nice to crop out most of the beach and the cars on top. The result is the focus on the wrestlers in the middle and the contrast of the colours and the yellows of the sand and rocks in the back.
I actually crop a lot in my pictures and I'm very happy with the possiblilities that digital photography offers in that sense. On the other hand I do want to work on framing the image in the viewfinder, so that I can actually decide at the moment of taking the picture what I want it to look like, and not with post editing. But for now, this is a great way to practice finding the right frame and exploring the design of an image.