When I was at my peak of my shooting skills I would spend about 45 to an hour every day standing 3 yards from the target eyes closed really concentrating on a perfect release. I would do that for about 30 mins, the rest of the time I would open my eyes and look at my pins but not aim at anything and focus on the perfect release. Every few days I would break out a 3 spot target and try to shoot a 300, I would start at 5 yards once I shot a 300 move back to ten, then 15 and so on out to 20. the most important thing is not to cheat your self. If its not a true 300 don't move back period! I didn't shoot longer distances than about 15 yards for close to a month. Boy when I went to the range the difference was unreal!!! Yes it is really boring but it will help your shooting like you would not believe.
P.S. Shooting a 300 is tough even at 5 yards. If you don't believe me give it a try.