Rick and Dave are right lots of pratice. Shoot the highest resolution, and in raw if possible.
People always want to shot at a lower resolution because if will give them lots of shots. It does not do any good to have shots of poor quality that you can not use if you get a keeper. case in point. I lowered my resolution because I am taking a lot of garbage shots trying to figures out a new gps toy. As luck would have it this guys jumped up on a rock in my area. Had I been shooting at full resolution I would be a to clean the photo up better. i know better than that.
A great forum to go to is DPREVIEW, this is th nikon link.
I hope I am allowed to post this link here. There should be a forum link to your camera type, plus a few general talk links.