I see that this is an old thread, but as a new shooter I wanted to update with my recent experiences. I just got back from my second year of high-power and feel like I have a better understanding of some of the advice on this site.

First, the basics are all the same. Learn each position and practice it. BUT, there are subtle things that you can do to improve your shooting. These are the "secrets" that the old guys don't give up as easily. With that in mind, here are some of my "lessons learned."

PREPARATION and GENERAL SHOOTING
Always blacken the front post. (Never the rear aperture. I saw a guy do that and wonder why he couldn't see through it)
Load your magazines ahead of time, and mark them. If you have a malfunction with the mag feeding, you then know exactly which one caused the problem and can take it out of service before the next event.
Mark your sling for the sitting/prone length. After offhand, you will be able to set it quickly and use your time on better things (like natural point of aim)
HYDRATE and BREATH!!! This cannot be emphasized enough. Drinking lots of water and gaterade will help your focus, vision, concentration, heart rate, fatigue, etc. Breathing keeps that oxygen level up whcih affects the same things as hydrating.
Use a blinder over your non-shooting eye, and a hooded sweatshirt or other visor to block the light from glaring in on the blinder. It's amazing how much this helps your sight picture.
Use the same anchor point in your shoulder every time for each position. The point will move with each position, but in that position it should be the same for each shot. PLACE THE STOCK on that point.
When lining up the sight picture, pay attention to reference marks on the front sight. You can mark with a pencil on it or use the base and wings. There can be a lot of variation in your shot of you line the top of the post up in the center of the rear aperture, but if your eye is off jsut a bit, rifle canted or tilted, etc. your shot will be off. It may take you a while to realize why it is off, especially if you think you are getting what looks like a good sight picture.
If you have your own rifle and shoot a LOT, watch your groups over time. I have been told that if your groups are climbing over a period time that it can be caused by carbon build-up in the gas tube. The build-up prevents as much gas from cycling the bolt, which can cause cycling errors and jams. It also will cause that extra gas to push out the barrel, which can increase your bullet speed and move your groups up. Along this same thinking, if your groups are dropping down the target you should make sure that you are cleaning the sight black off the front post. If you don't, and it builds up, your post will then be higher and your shots will be placed lower.

OFFHAND
Make sure that your feet are shoulder width apart. I was slightly farther apart, felt stable, but was all over the place. I moved in just a bit and tightened the group right up.
Relax into the jacket. It feels like your mush, but the jacket provides the support and relaxing your muscles takes the pulse and shake out of the equation. Have the bottom straps tight. They are more important to support than the top straps.
Even though you are relaxing into the jacket, keep your support elbow tight into your ribs. The more you tuck it in, the more stable your shot.
Tighten the sling up enough so that it doesn't hang below the barrel. Every little movement that you make will be exaggerated by the sling.
Wait to chamber a round until you are ready to shoot it. My first year I would shoot, reload, and wait for the pits to run my score backup, me to record it, and then re-establish my point of aim. I later found out that the round is heating up in the chamber and that this causes the powder to burn differently and the bullet to fly differently. It can throw off your group and cause confusion if you are not expecting it.

SITTING
Make sure that you undo the bottom straps of your jacket. Additionally, I have been told to loosen my belt. Having your belt the same tighteness to hold your pants up causes your belly to "balloon" over it when sitting. This adds another point of instability to your group.
Make sure that your elbows are IN FRONT OF your knees. My first year, I had my back elbow on top (thinking "bone-on-bone") My group looked like a comet tail streaking from the 10 ring straight down to the right. In the pits they could tell that my elbow had fallen off my knee, I just wish that I had noticed.
Also, have your leg that is closest to the target in front of the leg that is away from it. i.e. a right hand shooter has the left leg in front of the right leg when crossed.
When you squeeze the trigger, let it back out to the reset point. I have been told that I squeeze the trigger and hold it until the next shot, at which point I reset and squeeze. My rapid fire groups are ok, but I am not cleaning it so I will try this advice. There is a ton of time between shots to reset everything.

PRONE Rapid Fire
With prone, practice with one leg up or both legs flat. Pick your preference and stick with it. I have tried both, and currently am more comfortable with both legs flat and straight. On that note, if you shoot this way you should remember to always keep your heels on the ground. If may not seem like much, but if you drop into position and keep your feet balanced on your toes your whole body will move. I found this out the hard way. My first time ever at the 600 slow fire I shot GREAT. The next day I was up I could barely hit the target. My score keeper offered that advice after the match, since he saw me shoot both times and noticed the change. He was right.
Try keeping your support arm under/in-line with the rifle. I used to shoot with a lot of pressure on the sling and moving my arm forward and in-line has helped with groups and comfort.
It is ok to keep time on the rapid fire strings. I usually stand with the 2 round magazine and my watch in my right hand. They give the command to load, and then I am free to hit start on the timer when the targets appear. When you do your mag change, glance at the time. You will notice that you probably have at least 40 or 50 seconds left to shoot the 8 remaining shots. This helps with calming you down and making sure you roll back into a natural point of aim.
Don't worry about your scope on the rapid fire strings. At 200 you MIGHT be able to see the 223 holes, and at 300 it's a waste of time. Concentrate on position and natural point of aim.

PRONE Slow Fire
This is where you master the position that you use in rapid fire. Pay attention to sling tension, and body position. I have noticed that with my feet and legs flat, raising my body up just a bit helps eliminate some of the breathing effects. With my leg cocked up, there is much less breathing effect, so I get a little lower. The key is to find what feels the best and produces the most consistent results, then do that every time.
At 600 yards, take a second or two to watch the wind. You can feel it on your face, watch the flags, read the mirage, etc. Make mental notes on what your wind settings are and what conditions they are for. Someone said to stop messing with the book to record your shot if the wind is cooperating. I might agree for one or two rounds, but that book is the only thing that will tell you what you did later. It shouldn't matter if the wind is blowing, gusting, or gone. If you are paying attention to it and shooting with the correct dope on the gun, your round will perform the same. I was able to coach a scratch team this year, and I learned a LOT from watching the vapor trails and reading the mirage. If the shooters do what you tell them, you get instant and direct feedback on how you read the wind and how they are shooting. I HIGHLY recommned that you go to the range with someone that can shoot while you spot and vice versa. That exercise alone will give you a lot of valuable data. You need to be at least 300 yards to see the vapor trail and use it. You get the most feedback from mirage and vapor trails at 600+ yards.

In the PITS
Good pit service is valuable to the shooter. If you want it, you have to give it. Some things that may help:
Bring a length of rope to the pits. You can tie it to the bottom of the carriage and use it to pull down and up. This will save your back and speed up your pit service times.
Bring a spray bottle with vegetable oil in it. Use this to lube up all the rails in the carriage. It will slide much easier and make your time a little more pleasant. Some people use WD40 or other lubes, but if you travel to California or another of the communist states, you will need eco-friendly and biodegradable lube. Vegetable oil is cheap and it works, AND it is ok with socialists everywhere.

And in seriousness, I would go with this.

When you are in position, if something does not feel right, it's because something is not right. Break, rebuild your position.

It's your prep time. Prep time should have just as much of a routine as hot line time. Use it well. Practice it.

This is very good advice.

Spend more time shooting and practicing than agonizing over the minutiae of handloads and buying expensive equipment. I got my Expert classification on a reduced course of fire using a carpet mat, a camping stool, a home-made cuff sling, a $60 WalMart spotting scope and no shooting coat while shooting a 60 year old milsurp rifle and cast bullet handloads. Proof to me that HOW you shoot is infinitely more important than WHAT you shoot.

GG

You are not shooting to just put lead down range. You are shooting to find something within yourself. A capability, a commitment, a focus. That's why we shoot highpower, don't show up to a practice, a match, or anything else thinking "ok how am I going to shoot?" You are going to shoot beautifully, show up to the match and begin to focus on how you are going to THINK. When you can remove yourself from your present thoughts, that is the beginning of clearing away the cobwebs and your shooting will improve as your mental peace does.

Do everything the same way once you figure out what works, do it a lot. Make the action secondary to just being there. Wake up in the present moment being at a stage of fire knowing everything is ready (magazines, sight dope, position) because it always is, and because you couldn't have gotten there without doing those things. Make them totally second nature. Then just clean the target.