Disable mouse accel, find a sensitivity you are comfortable with.
Mix and match your sens, DPI and zoom sensitivity.
Don't be afraid to take shots, you won't hit shots you don't take.
Force headshots, i.e. aim for the head.
Don't rage when you meet a better sniper than you, there will always be someone better.
Instead, learn what you're doing wrong and what they're doing right. It will help a lot with improvement.
Don't get into a habit of doublescoping or afterscoping, sure you'll get kills, but you won't when you stop.