If you own an M1 Garand, you need to install a proper M1 Garand Sling on your rifle. You will find that with a good sling and proper shooting form, you can increase the accuracy of your rifle considerably.
This increased accuracy comes from being able to hold your rifle more steady and also prevents muzzle rise from taking you off your target with follow up shots.
With the M1 Garand being popular in high power rifle competition, this may be the most inexpensive addition to you rifle that increases accuracy so greatly that you notice it the first time at the range.