#1. Aim for Grav Pods behind a target. Unless you have a headshot lined up, this is often the way to go. Two or more grav pods can quite often result in a kill. Just make sure the Grav Pods are actually next to a target, not just off in the distance.
#2. Strafe left and right. Not only does this help you avoid enemy fire, it can also help you line up the target, without having to touch the right analog stick (sometimes, when aiming, if your finger twitches even slightly, you'll miss the shot).
#3. Only attempt to hit the body when they're out in an open space. If they're near cover, they may duck behind it before you have time to fire another shot, and someone else will get the kill. If they're near cover, go for the immediate kill (headshot).
#4. NEVER become so pre-occupied with a target that you focus on it to the exclusion of everything else. If an enemy dissappears around a corner, chances are, it's not coming back. Don't wait for it, just find other targets.
#5. Try not to snipe from an open place. While you're sniping, you have no radar, and can't see much around you. Sitting duck.
#6. If you need to jump to get a target, it's usually not worth it. Another sniper who wasn't aware of you may see you jumping, and kill you when you next popup. Besides, it's hard to aim while jumping: Not only do you move vertically, but when you touch the ground, the target will have moved, denying you a clean shot.
#7. Reload regularly. Only too often have I seen some poor newb try to snipe an easy target, but missing and having to reload, by which time the target is gone.
#8. Pack a close combat weapon. Chances are, somebody's gonna sneak up on you for an easy kill, and attack you. If they don't kill you first off, you don't want to be stuck with two long range weapons while they're dicing you up with an SMG.
#9. Look around regularly. Even if it's just a 5 second break from the scope, that's often just enough to tell if someone is nearby on the radar, or some ammunition has respawned for you. Besides, you can see much more when not zoomed into, which allows you to identify new targets ripe for the picking.
#10. Never waste ammunition. If a guy is zooming around unpredictably in a ghost, and you're relatively new to the world of sniping, don't try and get him. Find an easier target, and bring them down. The more ammunition you have, the more people you can kill. Sure, if you get that guy in the ghost, it's a good achievement, but it's still only one kill. Why waste five shots on him when you can kill someone else with one?