What improves my bench rest groups in no particular order:
1) Make sure your bags/rests are stable and allow easy adjustment.
2) Pick targets that "fit" your sighting systems. Sometimes I get smaller groups with a larger bullseye. Experiment a bit.
3) Remember breath control is very important.
4) Trigger control is very important. Also the stock triggers on most ARs are pretty bad. The Giessele trigger mentioned above is my favorite.
5) Do not expect accuracy if you are shooting M855 "green tip".
6) Make sure your sighting system is squared away. No lose stuff! Do you need to loctite stuff?