There's almost no limit to what you can do to tinker with a photo after you've taken it, but that doesn't mean you shouldn't also pay attention while you're taking the shot. The simplest way to improve your photos? Get closer to what you're shooting.
Helen Bradley makes the point well on her photography blog:
If there is one technique most digital camera users can use today to instantly improve their photos it is to stand at least two or three steps closer to their subject. Most photographers stand too far away from their subjects so the subject ends up being very small relative to the rest of the photo.
If you're a regular photographer, this might seem obvious advice, but judging from most of the Flickr streams out there, it's not always advice that's followed.