1. When trying to hit a moving target predict their movement and place your crosshair where their head will be.
2. Don't stay fully scoped in 24/7. When scoped in it limits your vision on the battle taking place around you and can result in your death.
3. An uncharged headshot (A headshot without any or litte charge) can be better than an fully charged headshot at times. 5 out of the 9 classes can be killed with an uncharged headshot with the classes being Scout, Spy, Sniper, Medic, and Engineer.
4. If a headshot is not possible or it is a tough shot to make a fully charged bodyshot can also be useful. The same 5 out of 9 classes can be killed with a fully charged bodyshot.
5. Don't underestimate the power of your SMG a nice bodyshot paired with the spray of your SMG can bring down any class.
These gameplay tips along with some practice will make a sniper out of you!