If a target is moving, make sure to lead it. If you are not making shots, try to lead your target more. Also, if you are a skilled sniper, try using the M40A3 over the M21.The slower reload time and rate of fire area small price to pay for having enough damage to kill a target in one shot instead of two or three. It also allows you to be less noticeable with less rounds being fired, so someone else trace your shots back to you. A good setup for the sniper is the Claymore perk to alert you when an enemy is trying to close in on your rear, as well as either the Sleight Of Hand or Stopping Power.Iron Lungs is also great as it gives you more time to line up your shots.
Choose three or four sniping points at every map through which you can rotate if one or two of your spots are "behind enemy lines", are taken by another sniper, or do not offer desirable shots.
Take advantage of original locations while avoiding the generic spots that were designed as sniper positions on the map. Other skilled snipers enjoy checking these spots on a regular basis. One example of a spot everyone checks is the uncovered bridge on Overgrown.
Don't always go for headshots. As a sniper you will get them, but only going for headshots leaves you with fewer kills and no ammo.
Do not camp in games that allow Killcams as the player you killed will most likely be out for revenge on his next life.
Find spots that provide cover (for example, not the middle of the road lying down). Sniping from extremely exposed locations makes you easy prey for more skilled snipers.
Pay attention to your body position. Although you cannot see them, your legs extend well behind you when lying down. Make sure you lay down so that your head faces your cover and your legs extend straight behind you and not off to the side. Prone snipers can be killed by shooting them twice in the foot.
When using a Bari 50 Cal., avoid pointing at an enemy then clicking as fast as possible. Shoot once and wait for the effects of the recoil to wear off before you shoot again, otherwise you will shoot straight up.
Perseverance is key. It takes a while to get used to the feel of your gun, the map, and the enemy. Experience is extremely valuable for a sniper so don't be discouraged if you're not tearing it up your first games sniping.