[Dire]

I went from a pretty mediocre to an decentish (level ~35) Halo 3 player. Biggest thing outside of the fundamentals (aim/predicting where a target is going to move to while aiming at them), was the maps. Learn them, memorize them. Watch videos of good players and follow the same paths they follow on the maps, and see why they follow them.

It sounds so simple but the biggest thing you really have to focus on is making sure your enemies are always in front of you, and your allies / 'safe' territory are behind you. Serious, once you get that down - you will be miles ahead of an average player. You'll be the one ambushing people left and right and rarely finding yourself suprised or confused about where you're being attacked from.

A good example might be the level Isolation on Halo 3. You'll always see bad players randomly scurrying about the top just trying to find somebody to shoot. The good players will always be up on the conrete slab looking down at everybody with their back protected, or coming up/around the sides unless they already have an isolated target and are going straight for him.