Most hunters know enough about their gun that when it starts shooting poorly they will either check their scope and scope mounts, or they will declare that after 1000 or so rounds that they have "shot out their barrels."
They don't stop to think that all those years of being taken into the woods through rain and snow storms and all the changes of temperature and humidity have had a major effect on the wood of their stocks. Given enough time, the wood will start to warp. When this happens your barrel and stock will lose contact with one another.
The quickest and easiest way to restore your rifles accuracy is to glass bed it. The three point bedding system utilizes two bearing surfaces at the forearm tip, the recoil lug, and the tang area. Personally, I prefer full length glassing of the forearm.
There are a number of books printed on the subject of glass bedding and a number of company's that manufacture glass bedding materials. Personally, I use my study unit from North American School of Firearms as my guide. I use acraglas bedding compound sold by Bob Brownell of Montezuma, Iowa.
So if your favorite rifle has lost some of that accuracy, before you retire it, check your bedding it could be a mess.