Snipers should always aim for the head. That way you can quickly dispatch enemies with little health like the scout and engineer, and thus remove some of the pestes that always bite your team in the ass. The sniper rifle loads up to full power when you are in the aim mode, and a fully loaded siper shot will kill any soldier, demoman or medic even if you just hit them in the body. A heavy is slow and you should always go for his head. Spies like to hit you in the back with their knives. This is annoying, but resist the urge of putting on your razorback. Then they will just shoot you instead, and that makes it even harder to kill them. Personally I use the jarate. This can give your team the upper hand in a fight, and it will reveal any spies you manage to hit.