I second the Scott Duff book.
-I use Tetra, the little yellow GI greasepots or white silicon grease--whichever is closest to my hand. I like to apply it with an acid brush, cycle several times, then wipe off most of it.
-Support the gas system, not the stock, when you tighten or remove the gas plug--use a gas cyl wrench or place the gas cyl lock in a padded vise that just barely makes contact. This will keep the splines on the barrel tight.
-Use penetrating oil overnight (Marvel's Mystery Oil or Kroil) to loosen that stuck gas plug. Patience won't damage the rifle like excessive torque will.
-Clean the barrel with the rifle upside down, muzzle slightly downward. This will keep solvent out of the gas system and out of the stock where it will tend to deteriorate the stock fit.
-Use a muzzle guide and extra care when you use a cleaning rod. Barrel steel is soft and the muzzle crown is the most important part of the barrel for accuracy--it's also very delicate. This is one of the hazards unique to a rifle that must be cleaned from the front end and a lot of people are unaware of the risk.
-Before firing, check that things are tight--gas plug, suppressor, rear sight tension (in the M14 field manual, FM 23-5 under the heading Regular Maintenance).
-Paint a witness line across your gas plug/gas cyl lock so you know how far to tighten the plug
-After you sight in the rifle, count how many clicks it takes to bottom the rear aperature, then write that number down (I have a sticker under the hinged buttplate). Even if you fiddle with the elevation drum, the number of clicks up will always be constant for a given type of ammo.
-From 100 to 200yds, add 2 clicks. From 200 to 300yds, add 3 clicks, from 300 to 600yds add 12-14 clicks ("two for 200, three for 300, twelve for 600"). This will put you on paper for any ammo you can feed the M1 or M14.
-It's not a bad idea to drop the hammer and unlatch the trigger guard prior to storing--it will help minimize the compression of wood around the receiver. Also, after oiling the barrel, remove the excess with a single dry patch. This will keep oil from running down into that sensitive area of the stock.
-Always focus on the front sight, not the target (and don't shift your focus between the two either).
Additional good reading:
http://web.archive.org/web/20030705071825/http://www.jouster.com/lanestips/
Ty