Try Not to Start the 2nd to Last Fight in the Game
When you are down to three people left in the game, you and two others, things are going to be very tense. If you start the next fight, even if you win, the other guy will know exactly where you are and push on you. There will be a good chance you are wounded too.
If the opportunity presents itself where you can take out an opponent without taking shots back, you should probably take it, but move the second the fight is over, unless you have such a superior position that the last opponent will have to approach you in the open.
Learning to deal with that close circle, three man dilemma is the largest hurdle to winning a game.
Careful Reloading or Throwing Grenades at People That Can Easily Get Line of Sight on You
I've won games cause the last guy decided to reload to his MG 42 around the corner where I could hear him doing it. I've also won games cause the last guy decided to cook a grenade when I could see him.
You can get away with this stuff against bots, but living people will take advantage of it and kill you.