Rule 1. No automatic weapons. Even if they have a single shot capability, the temptation for many people, will be to open up on full auto and waste ammo.
That right there is one that I have issue with. At least you didn't say use a manually-operated gun (pump, bolt, lever), because this is a matter more of training and discipline than effectiveness. If you could get your hands on them, two or three belt-fed automatics would be an absolute life-saver in case of a big horde, if you had a small angle. Have each person aim at a few inches different level, and sweep back and forth on that level as they fire.
As long as they can manage the recoil, that would cover all but the most extreme differences in height, allowing at least one of you to get a headshot on anyone that isn't a midget or Andre the Giant. Andre would be pulling himself towards you on his hands, due to having his spine severed by repeated sweeps, and the midget would be walking on corpses before long, making him tall enough to get shot without changing the field of fire. Alternately, have a fourth person taking shots at anyone still moving, or outside of the range of the fire. The main difficulty here is getting your hands on three belt-feds.
Rule 2. Keep a secondary weapon. A pistol of some sort, a .22 is ideal, small caliber, light, accurate at the distance for which you will need it, and no where near as noisy as a 9mil or a .45 or the other more common calibers.
.22s are good. Ammo is light, extremely common (it is frequently packaged in boxes of 500 or more), it is accurate as far as it will shoot (you can hit out to 200 meters with it, though holdover would be a pain, and I wouldn't trust it to punch a skull), small game can be hunted with it for food, but one thing about that bit bothers me.
Guns are loud. Period. Doesn't matter the caliber, they are loud. Get the supplies to make suppressors the first chance you get when you know the zombies are coming. The Anarchist's cookbook has a diagram of one, but that one is outdated, look on the internet. Find a way to attach them to your gun, if it doesn't have threading, improvise. It's even better if you have an integrally suppressed gun, but those aren't common, except among the most enthusiastic of gun collectors. Brits and most non-USers have the advantage here, actually, they don't have laws against suppressors (I believe they are sometimes called mufflers over there, like on a car). If you are in the US, get your hands on one or make one as soon as you can, if you live long enough to be charged with illegally making a class III firearm, you have already succeeded.
Rule 3. You need a melee weapon. There is no substitute for skull splitting or cracking power when it comes down to the crunch. A small metal club, or a mchete, or even some sore of customised fist weapon if you can get it.
Melee weapons are good, though your rifle's stock should be capable of doing the job, unless you have a nylon stock. What is a customized fist weapon? A gauntlet? Punch dagger? I like the idea of a gauntlet, but it would be hard to find, and probably a bit awkward to operate the rifle with it. Alex's Hooligan bar idea is a damn good one, I also like the machete, hatchet, or kukri (if you can find one), you will need to cut wood to survive when it gets cold, unless you are staying in a city. The aforementioned weapons also are more designed for placing the force of the impact in one place, making it more effective.
Rule 4. Keep water. Rule of threes, 3 minutes without air, 3 days without water, 3 weeks without food.
Rule 5. Leather is your friend. It's light, and very difficult for teeth and fingernails to get through if you have the genuine article.
Good rules, though you wouldn't be at your full capabilities for the last one (or even two) of those threes. Leather is very good, it doesn't matter if you look like a gay biker, the zombies won't care about how you look, just how difficult it is to get to your tasty flesh. If the zombies rely on sight, see if you can get some form of camouflage clothing over the leather, and learn how to use it well. A ghille suit would be even better, assuming you would be still, but those get hot as hell, and take a lot of work.
Rule 6. If you have to engage, do it from the nearest high ground, it provides a good vantage point, and zombies are poor climber, any high ground will do, even on top of an SUV.
This one, I'm assuming you mean if you aren't able to retreat while engaging, otherwise distance is your friend, it prevents you from being surrounded. If the zombies make sounds to attract others, though, take them out before they make a noise, if at all possible.
Rule 7. Keep your eyes and ears open at all times.
Rule 8. Try to make the best use of your melee weapon, ammo will be hard to find in a post apocalyptic world.
Rule 9. The final rule, if it's shuffling and doesn't talk, shoot first and ask questions later.
Pretty good rules here, rule 7 I agree with whole-heartedly, rule 8 I think largely depends on the person. I happen to know some people have ammunition stockpiles that would make Burt Gummer green with envy, and nice secluded houses, or friends with secluded houses, and trucks to carry all the ammo. Those people, who have hundreds of thousands of rounds, they are probably fine shooting everything. Everyone else, kill it without shooting when it won't put you in much danger. Rule 9, I would be hesitant about, but that is why I will end up bitten for being too nice of a guy.