What Sean said is true - try practicing at longer distances then come back in and I bet your groups will get tighter. Also, keep shooting, those repetitions are money in the bank, provided your form is good. And, shoot at the smallest feature you can see, as was also said, aim small miss small.
It may seem like you aren't improving for a while, and it is true that you can hit a plateau, but then you break through and get better.
The other way to improve your groups is better tuning for your arrows. The better your form gets, the better you become at telling if your set up is well tuned. Borrow or invest in some arrows of slightly different spine, material and set up. You might be surprised that you what you can gain from fine tuning your arrows.
Also look at the diagrams and explanations that Terry has up on the shooters forum. I go back and review these periodically and continue to learn and improve.
In the end, with unlimited practice, you could end up like Byron Ferguson who says that if he can see it, he can shoot it!