I didn't realize a habit of mine until last year when I taught my photography class at the university, but I have certain things I gravitate towards when I take photos. I really like to look for shapes, lines, symmetry, and color when I take photos--my eyes just naturally go to it. When you take photos, what do you naturally look for?
The photo above is at a park in Houston that I took this past summer. I wanted to fill up the entire shot with lines, so I got pretty close to the subject and filled the frame.
I think one of the biggest mistakes of photographers is not getting close enough to their subjects. If I would have taken a couple steps back, I wouldn't have been happy with my outcome. I guess I could have cropped the photo afterwards to what I wanted, but I think that you can accomplish what you want to without having to go back and photoshop your photo.
So, don't be afraid to get close to your subject--fill it up!