AUTO-AIM and LOCK-ON CONTROLS
An abomination. It shouldn't even be on here. Oh, wait...we like Halo 3, don't we? Okay, then, fair enough...
Auto-aim and lock-on aiming - as seen in shooters like Dark Forces, the Syphon Filter series, the Halo series, and the most magnetic crosshairs yet: Serious Sam HD - is the simplest form of accuracy improvement.
There. Next!
WEAPON SELECTION
I'm not going to sit on this one long, because it's the most obvious. You pick up an machine gun, you make a mess. You grab a shotgun, you make the same mess but in less than one second. The slower the fire rate, the more manageable your aim becomes. This is a basic principle of video game shooting.
"We need weapons."
CONTROLLED FIRING
Just like in real life, mashing down the trigger of an automatic weapon will make you an appealing target who's easy to find. Some lucky players use this method...more serious shooters call it "spray and pray". The idea is: you quite literally point the muzzle in a direction and fill the screen with lead. No matter how good you are with the crosshairs you will miss, and you will miss a lot. Using your finger to fire shorter, controlled bursts is the generally best way to ensure that more bullets land where you tell them to.
ZOOMING
Ever since sniper rifles invaded first-person shooters there has been no shortage of campers who like to stay prone and take a single, measured shot for the kill. As sniper rifles have become more prevalent, even the run-and-gun style players have resorted to the superior scopes of high-powered rifles every once in a while. Today's graphics render camouflage - like ghillie suits - useful (finally). Also, the addition of smoke grenades and weather effects help to mask your enemies. Getting a closer look from afar usually aids you in finding that prick spawn camper and waxing him.
CROUCHING
More appropriate for third-person shooters but present in most first-person combat games also, the crouch function serves a dual purpose: making yourself a smaller target, and steadying your aim. If the map provides low edges and objects, you can hide behind them...all while benefiting from a smaller bullet spread. Sometimes, crouching will also slow your movement speed, allowing more precise shots as you creep around corners. Which brings us to...
SLOWER MOVEMENT
Using your character's legs to more slowly move your crosshairs usually gives you the freedom of a slow target acquisition without sacrificing your character's quicker upper body movement. In other words: you can aim better, but still swing around if someone approaches you at close proximity. At the same time, slower movement balances multiplayer gameplay by presenting you as an easier target.
And finally...