Always rest well. Take a good sip of tea/coffee before the game, maybe train a bit in other modes (don't ignore them!), and only after that start a match. Insanely important after unlocking Ranked.

Fix Your Crosshair Placement: You've probably heard this a hundred times, I know, but how do you do it? There's only one way: PLAY A LOT OF DAMN DEATHMATCHES. They are the best way to be in constant fights that help you understand at what height the enemy's head will appear. In any case, almost all maps and common spots have hidden 'signs' that can help you figure out where to aim.

Graphic Settings: This might seem trivial, and you might think, "Oh, I have a NASA computer; I can play with high graphics." SPOILER: It's not that simple. No matter how good your computer is, you can always improve input lag. Make sure to: Set the in-game resolution to match your monitor (usually 1920x1080). Disable VSync, Vignette, various shadows, and set all settings to the minimum, except for textures and models, which should be set to medium. Use 4x Anisotropic Filtering and 4x Anti-Aliasing. In the general settings, enable 'RAW INPUT (BETA).' This allows your mouse to bypass Windows and go directly to the game.