Crop

[jacsul]

Would this be considered a good crop?
This

From this

 

[Lipoly]

I think the original could use a crop, but generally you don't center a moving subject...leave some "extra" in the direction of motion for somewhere for them to go (to the left in this case).

 

[reznap]

I think you cropped way too much. The background is actually nice, no reason to crop it all out. Here's how I'd edit it if it were my photo:

 

[KmH]

There are many compositional guidelines. One of the most used is called the Rule of Thirds (ROT), and is best accomplished in the camera when the image is made.

ROT illustrated: