Bhai:
Burst and tap:
You have to slap yourself (in real life) whenever you crouch and there is an onrushing team. The best way to stop an onrushing team, on lan atleast, is to keep on moving left and right and shoot only 2 or 3 bullets at a time. Like move left, shoot 3 bullets, then move right, shoot 3 bullets. Keep doing this and you'll practice.
A way to improve stretching aim is to make two points on a wall on CS far away. Then move your mouse quickly on those two points, shooting one or two bullets on one point, then move mouse quickly, then shoot 2 bullets on that point. Keep doing this for 5 to 10 minutes every day and you'll start to know how your mouse works and how much you have to move it for it to reach a certain destination.
For Ducking:
Keep running forward or backward and keep pressing CTRL like this:
CTRL..CTRL............CTRL..CTRL............CTRL..CTRL.... and so on. Press two times quickly, and then wait for 0.4 or so seconds, then press two times again quickly.
Practice it.