Fall damage reduction

To achieve a Fall Damage Reduction, play on any level in Halo: CE. You can jump off a large cliff and land on a steep slanted surface, one that you wouldn't be able to stand on if you were down there. The reason for this is that there is no friction on those surfaces. If you do it right, you will not get hurt. Usually, it works best if it is a steep, slanted, rock, or a Wraith. Although, if it is too far, you will die. A good example of this is when you are on a bridge. Another way that worked in was to tap "Y" really fast and it would work on small ledges.

Walkthrough
An example of the Wraith is on the level Halo, when Cortana says to go to the third level, you can jump down and land on the front of the Wraith and you will not take damage. This can be used to shortcut some levels. During the level Two Betrayals, when you are going down to get the Banshee, you can slide down the slanted portion of the walls and effectively skip the walk down.

Another example is on the level Keyes when you are on the third floor and have to get to the Banshee, jump down onto the second floor, kill the  Elite in the Shade and try to jump onto the Banshee to avoid fall damage, avoiding all the Flood/Covenant the long way offers. If you get into the Banshee immediately, you will skip to a cutscene and avoid fighting the two Elites there.

An additional way to avoid fall damage, specific to the Halo 2, is to corner ride. A technique developed by the Halo glitching community whereby the player repeatedly motions back and forth over a 90 degree vertical angle. This periodically 'tricks' the game into believing the fall timer has been reset, preventing death.

Probably the trickiest way to avoid fall damage is crouching. With good timing, if you hit the crouch button just before impact, your shields will take little or no damage at all. This skill should probably be experimented with before fully relying on it, as it is very likely for you to mistime it and be killed by the impact. If it works, it is very effective. However, split-second timing is only required to reduce all fall damage; otherwise, you will lose a portion of your health if done properly (ex. all your shields, etc). However, if gone horribly wrong, then you can actually die/take more damage than if you hadn't crouched, so be careful.