Shooting Offhand:
First off, Pardon the spelling, I had a secretary when I did this for a living.
First one must be comfortable, when coaching rifle shooters I'm always asked, "How far apart should my feet be". I ignor the quetion for a few minutes and have the shooter stand, and stretch. Then while they are standing I stop what ever lecture I'm doing, and tell them to look at their feet. (all are differant distances apart). Then I tell them that is excactly how far apart their feet should be.
Next: Bring the rifle to your shoulder, Depending on the rifle, the non shooting hand should be either under the mag. as in ARs and M14s, using the mag as a Palm Rest. If using a Garand or simular rifle, then the hand should be just a head of the trigger guard, under where the mag would stick out if there was one.
Bring the rifle to your shoulder, Move it up so you can see the sights while the head is straight and errect. The head shouldnt be tilted at all. If you have to push the butt higher up on the shoulder, so be it. Its got to be done.
The elbow of the non-shooting arm should be resting on your side, hip bone if it sticks out enough, this is where a large gut comes in handy. Regardless, the elbow has to rest on your body, there should be no undo mussle work holding the rifle.
Now, line up on your target. Close your eyes, then open them and see where you are in regards to the target. Make adjustments to elevation can be adjusted by moving the butt up and down in your shoulder, (keeping the head straight up) or by moving your non shooting hand toward the muzzle or trigger guard, depending if your pointing high or low.
For windage, move your trailing (back) foot, left or right to move you sights onto the target. Then put down the rifle, close your eyes, bring it back to your shoulder pointing at the target and open your eyes to see where you're at. Keep this up until you are on every time you bring the rifle up.
Most important. RELAX, that is one of the most under rated shooting fundalmental mentioned, yet its extremely important. Granted the further up you get (from prone to kneeling, to setting to standing) the harder its gonna be, but you can still relax.
Dry Fire: then Dry fire some more.
Use a score or data book, record all changes in your position and the results.
Now for firing. Load the round with as little movement as possible, if shooting a match where single loading is required, place your ammo where you can get to it without bending over. Have you scope set so you can see the target by just moving your head, not body or feet.
Front sight, just as important in rifle shooting is concentrating on the front sight with a rifle. Try to imagine the front sight is on a lever attached to the trigger, as you pull the trigger back, you are moving the front sight. Move the front sight slowly to the rear. Now we all know you cant move the front sight with the trigger, but, if your are doing it right it will look like you are actually moving the front sight. What is really happening, is your concentration is to the point the front sight is clear and sharp, making it look like its moving closer to your.
Shoot with both eyes open to prevent eye fatigue. If you cant for what ever reason, get a pair of shooting glasses and blacken out your non shooting eye. That helps.
Follow through: Extremely important. Best way to practice this is the imagine you are firing two shots rapidly, after the first shot, get back on target as fast as your can like you are gonna firea second round. Normaly people want to look at the target to see what happened as soon as the round goes off, what happens they start bringing the rifle down while the round is still in the barrel, (subconsiencualy). Use that to your advantage, coming back on target.
Target: Dont shoot for Xs or 10s, shoot for black, you aim for the black, (which should be fuzzy anyway) the 10 & Xs will take care of themselves.
Breathing: that you have to figure out your self, contrary what your read or hear, everyone is differant. You can tell in your dry firing when in the breathing cycle, the rifle is the most steady.
Off hand, (pardon the pun) this is all I can think of right now,I'm sure more will come. Go to the CMP web sight, and for $6.95 you can get the SERVICE RIFLE Guide put out by the Army Marksmanship Unit, the best shooters in the world. It will cover not only offhand but other positions as well.
Main thing is practice what works, dry fire and practice, spend time just standing there in the offhand position to help develope stringht.
and RELAX