I'm by far not an expert on this subject. I do know what has worked for me however.

1) Proper equipment! If you want good groups, your rifles and scopesmust be able to shoot that quality of group.

2) Like RR said, Form, Breathing, and Trigger pull need to worked on. Shooting longer ranges does indeed help.

I prefer a heavier rifle than most. My 243 weighs in at 8 3/4 pounds bare. The cross hairs are a LOT steadier in the field than a ligher 6 lb gun. Its made a huge difference in my shooting. A lot of folks don't like the 2 or 3 extra pounds. Come on folks, I get 2 to 3 pounds mud on each boot!! If you can not carry 2 or 3 extra pounds then you are going to have a hard time getting a deer out of the woods!

Reloading might also help your shooting ablities. Its cheaper and would allow you to shoot more. It would also allow you to find the right bullet and power combination for your rifle. There by increaseing accurarcy.