A couple more things that help tighten the group on a Garand:
1. Trigger job down to 4.5 lbs (CMP doesn't allow less)
2. Peen the barrel splines to tighten the gas cylinder on the barrel. http://thecmp.org/training-tech/armorers-corner/gas-cylinder/
3. Use the sling properly.
4. Darken the rear and front sights with a match, paper burning, etc. It seems a small thing, but it really helps.
5. Join a rifle club that has Garand matches and practice, practice, practice.
Before all of these I shot a 220-250 (out of 500). After I started listening to my club members, learned the positions, peened the barrel, got a trigger job, and started handloading for it, I improved to my best so far. A couple months ago I shot a 442!