Setting up your game

First of all, if your game is not setup properly you will have a lot harder time being successfull. Here we will go over a few things that you should implement to increase your chances of improving.

Weapons on seperate keys (no, scrollwheel does NOT cut it):

It's very important to have all your weapon bound to seperate keys. All weapons have counter weapons, and you need to select the correct weapon for the invironment you are in, for example if you are in a close area fight and you have the minigun out then you are dead meat if you meet a player with flak or rockets out. This means that you need to be able to go straight to the weapon you need in a split second, scrolling to find your weapon is a recipe for disaster.

As mentioned above, select the right weapon for the area you'r in, constantly be aware of your surroundings, if you head out in the open pull out the minigun, shock, sniper, weapons that are effective against players that can move around, if you head into close quarters, pull out rockets, flak, or load up your bio rifle, if you go into a long narrow corridoor pull out your shock rifle and get ready to do some combos, since a player you catch in the corridor can't dodge out of the way, learn to constantly think about your surroundings.

Also make sure you turn OFF "weapon switch on pickup", YOU need to be in controll of what weapon you have out.

Hidden weapon:

Almost all good players play with hidden weapon. When playing with hidden weapons, all you have to focus on is your crosshair, and that's where the bullets will go, having a huge weapon take up your screen will make it harder to focus on your crosshair.

Hidden weapon also makes you focus on weapon sounds and learn them, you will soon know all weapon sounds and how they sound when you switch to them, this means that if you hear someone comming and they switch weapons you know exactly what weapon they have out and can take counter measures: "hmm that guy just switched to stinger, we are in close quarters so he will be helpless against some rocket spam"

Your crosshair:

Selecting a good crosshair is vital. Yes, something as simple as selecting a crosshair that works for you can improve your chances a lot so experiment until you find a good one. Most veteran players use a simple very small pink dot. Pink, because that color is almost never in any maps so you will never lose sight of your crosshair in a fight or have it blend with the colors in the maps.

Here's 2 articles that shows how you can change your crosshair:

Custom Crosshairs
Change your crosshair color