The key to surviving in ArmA2 is simple: PAY ATTENTION. 99% of the time someone gets killed, it's because they werent watching their surroundings. Due to the lack of any advanced warning system (apart from radar on some vehicles and aircraft) the only way to spot an enemy is with your Mark One Eyeball. Watch for movement, pay attention to uniforms, and generally keep under cover and/or concealment at all times. If you hear a shot ring out, get low immediately and find cover. Never sacrifice a safe position for awareness. If you dont know where they are, at least you'll be in a spot where you cant get hit. Use your ears to listen for incoming rounds. If they're far enough away you'll be able to tell their general direction by listening to the crack of the round passing by and listening to where it hits nearby. Fight the urge to immediately run, but dont stay stationary. And know that the enemy is smart. They will not just run straight at you. They will flank. So keep an eye out to the sides, and even your rear.