Preparing for combat
When loading out for combat, take time to choose the right tools for the job. Basic sense comes in to play, here. Think about what you're going to encounter and prepare accordingly. A rocket launcher will do little in a setting with no vehicles. Take a good look at the map and figure out your plan of attack. Notice the location of objectives and consider how you'll get to them. The shortest distance between two points is a straight line, but the fastest way to get killed is to jump right into the fray. Take a look around and observe before going into the line of fire. Avoid confrontation until you have control. Pick your fights and try to get behind them.
Warfare is deception
All warfare is based on deception. When you're ready to attack, you must seem unable. When unable to attack, you must appear ready. In a skirmish, stay on the move and watch your back. If you ever lose track of an enemy, check behind you.
Loosing respect for the enemy is a fast track to getting sloppy; but that doesn't mean you can't look down on them, sometimes. Use every opportunity to come at them from an unexpected angle. This includes attacks from above, attacks from behind, and even attacks from below. Use explosives to breech open walls for a dynamic entry.
Do your best to catch the enemy off guard. Stay loose with your squad mates to appear alone. When an enemy exposes himself, put him down together. Do not pursue enemies the way they went. Lull them into thinking you've broken off, then catch them from the side.
Take advantage of what you find
A dead man's kit is a tool waiting to be exploited. Steal equipment and use it to your advantage. Place spawn beacons and motion sensors from fallen recons to use them as your own. Gather ammo and health if you find them. Borrow an engineers tools and scatter his mines about the ground as your own.
Dangers of choke points
I've seen more men die in doorways then I care to count. Always take care when approaching a door or turning a corner. Use every tool at your disposal to ensure you're not walking into an ambush. Be wary of claymores.
If you're careful and observant, you may come across a golden opportunity to breech and eliminate a threat all at once. If you already know an enemy is waiting for you inside, a well placed grenade may make short work of them. If it doesn't, you're would-be attacker may be injured, or at the very least, suppressed. If you listen well, you can hear enemies cry in panic about the grenade. Pinpoint in the room where their voice came from, and swiftly take them down.
Myth: the deciding factor in combat it fire-power
Reality: trading fire-power for mobility increases life expectancy.
Combat is decisive and you must be decisive. Your life, and the lives of those around you, depend on split second decisions you make. Be light on your feet, take deliberate shots, and, above all, keep moving; because you won't know you're in a bad position until you're stuck in one. When under fire, re-position and reacquire.