I'm not a real good shot, so I have to rely on tactics more than anything else. Here's some things that've saved my butt and/or got me kills. Feel free to add yours but try to avoid the ones that everyone's told before. Tell us YOUR secrets.
KILLCAM. Don't be the idiot that shoots somebody then stays exactly where you shot them from. Experienced playes know the maps like the back of their hands and will identify where you are from the killcam and come back to slit your throat. LEARN THE COUNT- next time you're killed yourself start counting and note your count from A) time of death to end of killcam and B) end of killcam to re-spawn. Then when you know your count use it when you kill somebody ESPECIALLY if you're near a flag because if they've got any kegs they'll be determined to re-spawn at that flag and get their vengeance on you. So when you kill them, use the time of A (death to Killcam) to feint - run to (or stay) where you WANT them to think you're gonna be and then use B (killcam to re-spawn) to get into the position that you WANT to be. You can set traps this way - there's the spot where you want the vengeful enemy to think you are, and then there's the spot from which you're watching the spot where the vengeful enemy thinks you are. So when he comes back to slit your throat, it's you that runs up behind him and slits his. Of course if you're not near a spawn point this is pretty irrelevant but it's a good habit to get into for when you are.
DEAD LEGS. Whenever taking shelter in a structure, DO NOT hide yourself up close to a wall and go prone (in fact I don't wether standing, squatting or prone). The reason is because if your legs or other parts of your anatomy are butting up against that wall - they'll stick out the other side wether it be through solid concrete or cold steel. And an enemy on the other side will see your legs and shoot you in the ankles until you are dead.
SPOTTING. "Just mash the back button" - anyone told you that? WRONG! The game has an anti-mash built in so if you mash you maaaaaay or maaay not spot the first enemy, but none after that. Slow, deliberate spotting giving the machine the chance to know you're NOT mashing is the way to go. You can still scan/spot, just be slow and deliberate about it. For an example of this in action next time you see a parachute, point at it and mash and you'll see you spot nothing, Now point at it and press once - enemy parachute spotted.
CAPPING I. If you're lucky enough to be in a squad with mics or running with a mate, when you all rush to cap a flag DO NOT all rush in there at once. One go in - if it starts capping there's nobody there, it don't there's at least one enemy there. So now bring in a second squaddie, if it's capping you know there's one enemy there, if it doesn't at least two. And so on and on with 3-4 squaddies working together. Knowing how many enemies you face is an advantage so use it. If you're real skillful (and I'm not particularly) you can tell how many enemies there are just by the speed of the capping so if you got this skill or are capable of developing it do so.
CAPPING II. On your own. DO NOT run into the centre and sit by the flag. Look for cover as close to the edge of the capping circle as possible and then defend the approaches - DON'T stay on the edge looking IN at the flag, stay on the edge looking OUT. The reason is that if you're right near the edge and it's capping, the flag itself has your back. Anyone enters the circle and you'll be alerted, so you can look outwards defending the approaches. If you're looking at the flag then someone can sneak up from the approach (if you're on the edge) and stab you in the back without you knowing they're coming. For them to do it while you're guarding the approach, they have to enter the capping circle and so you'll be alerted to their presence.
DEFENDING. Some may call it camping and I know it's not etiquette. But some maps do lend themselves to flag defense and I'm looking at the AK maps as I have an obsession of keeping the Gunship flags in friendly hands. So if you're guarding flags take a bit from the above CAPPING II. DON'T sit inside the capping circle as if one enemy enters you won't know he's there but he'll know you are (2+ you will if you're alone). Find defensive positions outside the capping circle where you can attack with first shot advantage if you see them coming, and if you don't then at least you'll know an enemy has arrived because the flag will start capping and then you'll have the advantage of knowing he's there while he doesn't know you are... untiil of course you step over the cap line and do your best defending it.
MINES. This is a tip I picked up from a mine laying guru on Youtube and it's a beauuuuuuutiful one - USE RUBBLE. Once things start getting blown up in game you'll find bits of concrete, assorted crap, even things that look suspiciouly like pizza boxes lying around the place. Lay your mines directly on them and sometimes (not all) you'll see that they disappear straight into them -and even when they don't you can position them so that they can't be spotted from the approach you expect the enemy to come. Some rubble you can even push around into position as demonstrated by the Youtube bloke but i never really bother with that, I just use the rubble where it lay if it's in a good enough spot. Done right the mines CAN NOT BE SPOTTED by the enemy. Also any approaches where there is water (EG: the landing zones on Noshair, the creek on Caspian), if you can stand in the water you can lay mines and again THEY CAN NOT BE SPOTTED by the enemy.