Counter-strafing is the single biggest unlock. If you're still moving when you shoot, your bullets aren't going where your crosshair is. Tap the opposite direction to stop dead, then fire.
Other stuff that actually worked for me:
- Pick one sensitivity and stick with it for at least a month. Constantly changing it kills your muscle memory.
- 15-20 mins of aim training before matches. Workshop aim maps or Aim Lab, whatever you prefer.
- Deathmatch teaches you gunfights way faster than comp queues do.
- Record a demo and watch it back. You will notice so many lazy peeks and bad timings in 10 minutes.
Ignore the people saying it's cooked past 25, people improve at any age with decent practice habits.