No-scoping

Description
No-scoping is the act of shooting an enemy with a scoped weapon, but without using your scope. Learning to no-scope is a definite must for advanced players. It will indeed save you many times when a charging enemy must be quickly neutralized when taking the time to go to the scope screen will waste valuable seconds and likely result in your death. It is a valuable skill to possess in a repertoire, especially when armed primarily with a Sniper Rifle.

Walkthrough
As with most Advanced skills, practice is essential to master this skill. So if you don't get it right away don't give up; just keep on trying.

When an enemy is approaching you with a Battle Rifle, M6G Magnum, or Carbine, don't waste time trying to scope. Just keep your cool and remember that you only need one shot, while they need a minimum of 7 rounds to kill you (with the exception of the Battle Rifle). Just line your reticule at head level, and if possible strafe left or right to line up the shot and fire.

That is the most common situation to use no-scoping. And since one of the best ways to counter a Sniper is with the Battle Rifle, and that's a pretty common situation to find yourself in.

Arguments Against No-Scoping
Since Sniper ammunition is very rare in the Halo games, each Sniper bullet is extremely valuable. Therefore, instead of wasting ammo on a charging enemy by using the no-scoping tactic, it would be better to switch to a Battle Rifle or any other weapon that can kill the threat, without wasting the "ever-so-precious" Sniper ammo.

However, while Sniper ammo is pretty valuable in the extensive maps, you want to keep the other team from getting your Sniper Rifle. If you have to spend two to three shots keeping yourself alive, it's often worth it in the long run if you don't let the other team kill you and take your Sniper ammo. The best players will often never get into this situation as they won't allow the other team to get anywhere near them. Even if the other team spawns near you, you should just move and migrate to a better location.

Furthermore, most players see no-scoping as an extremely, annoying tactic as it is abusing the sniper rifle and players tend to get just a little ticked off after being shot point-blank by a noob with a sniper rifle for the fifth time.

Tips

 * When you are practicing, don't just keep aiming for their head when you are completely under control and in an easy situation. Rehearse the no-scope shot under pressure, so when you are panicked you know you are capable of handling the pressure. Also, try to shoot them once, then melee if they get close enough. This is the best way to do it since you use only one shot and still get the kill. It's easy and doesn't waste ammo unless you miss the original shot.
 * Another strategy for no-scoping is jumping. Instead of taking the time to line up the shot on their head, get the target on the center of their body and jump. Pull the trigger as it passes their head on the way up or on the way down. It's almost like face painting but with no auto-aim. It works most of the time, as long as they don't move.
 * No-scoping is especially easy in narrow corridors, as well as effective. Sometimes players will take a Shotgun into a narrow passageways and just charge you to get close enough for the fatal shot. No-scoping with a Sniper Rifle is a great counter-measure to this technique. This is especially true for the shotty tunnels on Sanctuary.
 * When in a pressured situation, always tell yourself you will be able to pull off that no-scope. Being pessimistic will make it a lot harder, so always view your magazine half full, as opposed to half empty.
 * An easy no-scope is to put your reticule in front of a moving player. When the player walks into your reticule shoot him the head without using the scope to get your no-scope kill.
 * A good tactic is to quickly rise to the opponents head once with your reticule. Once that is achieved, "swipe" or "whip" to the left or the right and instinctively pull the trigger. The auto-aim will slow a bit; slow enough to the point where you pull the trigger and achieve a true "headshot". Remember: don't panic. The idea is to make your opponent panic.
 * In Halo: Combat Evolved, a good way to practice is to go to the level The Truth and Reconciliation. In Halo 2 you can practice on Outskirts and Regret. In Halo 3 practice at the beginning of The Ark.
 * You will have varying time frames to complete your no-scope, but generally no longer than a few seconds. It is important to find your shot and take it as fast as your can before opposition strikes and your chances at pulling it off heavily decrease.
 * You don't have to get headshots to be good at no-scoping. To get a no scope body shot and clean it up with the BR is as acceptable as a one-shot kill.
 * Usually, when you are trying to make a "No-Scope" head/body shot, its best that you fall back in a straight line, so when any player charges foward at you, he/she will usually do it in a straight line, thus locking on will be easier to get the kill.
 * A video of 9 consecutive no-scopes can be seen here: