If you follow these basic tips then your K/D ratio will definitely increase as well as the fun you will have. Dying is not fun. Winning is fun.
This list is based on COD: MW3 but most can be applied to any FPS.
1. Keep your gun up- Don’t run around with your gun in the clouds or aimed at the ground. If you always keep your gun at chest level then all you need to do is aim left or right. This will significantly decrease the amount of time it takes to get to your target.
2. Keep your back to the wall- A lot of people run-and-gun. That is totally cool and if you want to be Rambo then do it, but always keep your back to a wall or border. This will limit spawn points as well as assure you that you wont get killed from behind.
3. Use the right guns for the right maps- For example, if you like to snipe on Outpost then you should be using a thermal scope on your rifle as well as the snow camo.
4. Use the right perks- Everyone has different play styles which are complimented by different perks. If you are a run-and-gun type player you should be using offensive perks such as quick draw, slight of hand, etc. If you are a sniper (camper) then you should be using defensive perks such as blind-eye, assassin, etc.
5. The myth of silencers stabilizing your gun is completely false. This was myth-busted by a developer.
6. Join a clan or play with other people- Playing with other, good, people will really help your chances in winning and even your K/D. For example, if your party member runs left and you run right then you should be confident your party member will either kill everything in their path or tell you if someone is coming.
7. If you are accurate use a Type-95 for most maps- When I say, “accurate”, I mean 23%+ bullets hit target rate. If you like to spray your gun around then maybe a light machine gun is for you. Type-95 is insanely underrated as it can have rapid fire, is great at long range, and 1x 3-round-burst will kill any enemy at close to medium range.
8. Adjust your kill-streaks- If you have osprey gunner, juggernaut, or other high kill-streak rewards then you need to be a top player (2.0 K/D+ minimum). Otherwise, stick with rewards like: care package, predator missile, and the chopper. This will maximize your kills most certainly.
Make sure to comment on any other tips I might have missed!