It's good to learn when to take it slow, and when to run like mad. Many players are impatient. If you see someone but can't get a shot off, sometimes it's good to hide around a corner and wait for them to pass you. Then you shoot them in the back. Most people are in a hurry and won't check every nook and corner, and merely crouching in one is an easy kill. Just be sure you know someone's coming so you don't camp for the whole round waiting. Even if they do check your corner, you still have a good shot to win a head on battle because you were anticipating it.
Other times, it pays off to take a risk and run across an open area, then circle around on an opponent and flank them, which leads me to another important point: Never go straight to your objective. Since people usually spawn near the flags, people are looking in that direction for bad guys. For instance, if you are on Panama Canal and control C and D, and the enemy has A and B, you will want to approach from the direction of one of the uncaps, or run down into the middle canal, and shoot enemy soldiers from behind while they are moving from A+B to C and/or D.
Yes, I try to shoot everyone in the back.
The other thing is that you will start to see things better the more you play. After a little more time playing you will see enemies easier through foliage and rubble, and you will be able to look at the map and pretty much know where the fights are and how you want to approach or avoid them.