Be a Good Sniper in Any Game

Ever wonder why you will be walking down some deserted ally in your first-person shooter game and suddenly, someone snipes you in the head?

Steps

Find a good sniping spot. If you are getting killed and don't know where the sniper is, it means that there are good sniping spots somewhere.
A good sniping spot is not necessarily high up. Just find some place where you have good visibility and are mostly hidden
Hunt down anyone who's sniping you. Go back to the place you got killed from, then hide behind something. Find out where the sniper can and cannot see, and hopefully eliminate them. (either by sniping them or lobbing a grenade at them)
Find out how he got to their sniping spot. If it's some pro-gamer spot, then forget it.
Find your own spot. This should be somewhere hidden in shadows.
Keep moving. A good sniper never sticks to one spot, then everyone just thinks you're not smart. Move around so nobody concentrates on one spot so they can kill you.
If you are a sniper that is more mobile than others, try strafing more than aiming with your wrist when targeting an enemy. (using your wrist causes more jerky aiming)
Make sure that your back is safe. One of the worst things is when you're sniping, and about to get in a good shot, someone comes and kills you from behind. Make sure that you have a wall behind you, or you know that no one can get to your back.

On some games, the sniper rifle is 100% accurate but has no crosshair until you zoom in, so to no-scope find out the exact middle of your screen and place a piece of transparent tape and draw a dot where the middle is, then you have a no-scoping crosshair that is 100% accurate.
Make your character as camouflage-able as possible.
Try covering your body as much as possible. Hills, cliffs, obstacles/objects are great.
If your character can lean, try sniping leaning is effective and is almost impossible for the enemy to see you.
If you kill someone who notices you first, make sure to move as soon as you have taken care of them, as they will probably come back to kill you afterwards.
If your pinned down, run to another area (without the enemy seeing you) then engage them from there.
Get used to aiming for the head/hair of the enemy.
Try jumping so you expose your sight just enough to get a shot at an enemy when behind cover.
Get the fastest shooting sniper rifle. A gun that shoots slow but kills fast is not as good as a machine gun with a telescopic sight. However, if you don't have a 1-hit KO gun, your enemies can get a fair idea of where the bullets are coming from. Make sure you have a good shot before you take it.

Don't annoy people so much that they kick you off their servers for owning them.
Some people will dislike you for using 1 Hit KO Sniper Rifles and call you a N00B