Ok, some of these posts are pretty wrong, I'll make a quick version without inaccurate and useless junk and explain how it should be done. (And oh, by the way, from someone that uses every weapon, rifle still trumps all else by miles.)
1. Learn to shoot on the move- a rifleman that can headshot on the move is far more useful than a guy that sits in one spot and does very little. A rifleman that only holds a front is less useful than one that absolutely demolishes one, blows right over it and grabs the intel to boot. Cover is also overrated if you can completely take control of the fight on the move and deliver headshots to them regardless of where they are before they can even aim at you.
2. Maintain awareness at all times and watch your flanks. If you play like a boss, blow over every obstacle and leave your usually less than competent allies in the dust there won't be anyone to back you up. Awareness and reactions are your two greatest carries after solid aim.
3. Avoid low ground chokepoints where possible, especially intel pits and valleys. At the same time, avoid the highest terrain around. Those are the two main places anyone will be looking to dome you.
4. Try not to hit the space bar key. Your accuracy goes down the pan ridiculously fast if you jump. Only time jumping in combat is advisable is if you're skilled enough to look down the sights, jump above lower ground cover and dome a guy at the peak of your jump. I wouldn't advise it for the average player though, as its fairly tough to pull off for more skilled players, and has an unsurprisingly high failure rate.
5. Get used to hip fire killing. Its a lot faster at close to mid range to flick and dome someone than it is to deliberately aim down the sights to land a shot. Just don't do this over long range or everyone would laugh as you miss horribly and get domed from draw distance.
6. Just like aiming down sights, crouching increases your accuracy. For more reliable long range killing, sights and crouching usually results in a perfect shot.
7. People's weapons poke out when they're close behind cover. Almost any camper will be sitting as close to the block as he can get and his weapon will show right through like a beacon that says 'Please ram your spade deeper in me!' You can either shoot the guy's blocks out and dome him, flank him from the side, or if you're really feeling like a mean one, run up to the block and spade him through it.
8. Learning spawn timers is useful, and spawnkilling is a shameless tactic to bring victory. If you're dashing to the intel and a guy spawns in front of you, if he reacts fast enough you're toast. So don't try to be the 'honorable' man, just kill him. Or rather, wait until the next wave spawns, kill the lot and then carry on running.
9. Camping isn't dishonorable, its a tactic. Get used to it and utilise it. If you're quick enough to drop an enemy who is carrying the intel, you'll have a bit of time to set up a little staging point where you can trash anyone that comes into view. Just don't sit in the same spot time and time again or you'll be found almost immediately.
10. Grenades are a very valuable tool, especially to psyche players out. Spamming 3 nades in one place isn't always the best way to do things. If someone's behind cover, as a rifleman, your breaking capabilities aren't particularly potent, so grenades compensate significantly for this. Pop a grenade in and immediately charge right after the explosion, they'll usually be too busy worrying about not getting killed to react to a bullet to the face. The same applies to guys throwing nades at you. Retreating from a nade is a great way to get a bullet in the back, you have a much better chance of surviving if you run past the nade and go all-in.
11. Use your brain. If you can't hit someone's head behind cover but can see their feet, take the shots and whittle their hp down, if not kill them outright. If you think something's a bad idea, it probably is and you shouldn't do it. Remember, there is no 'sniper' in this game, just a rifleman and a multitude of ways to play him. Just learn to click heads and do so reliably in any situation, regardless of playstyle, preference or tactic, and shooting to the top of any server's scoreboard while 100 kills behind the leader shouldn't be a problem. Practice and speed is also key- the faster you can find an enemy, aim and shoot, the greater your chance of survival, so keep at it.
Most of the stuff here is fairly logical and usually obvious stuff, but I guess it just needs to be said among the midst of misinformation.