Crank up the Sensitivity
Right now you are at a 2, weíre gonna need you at about a 6. Experiment with a higher sensitivity, this allows you to more quickly spin around to watch both your back and your medicís back for spies or anything else that might hit you from behind. This can also help with tracking people who are circling around you, like pyros.
Pre-spins are like Surprise Butt-Sex
When rounding a corner or droping to a lower level, spin your gun and jump, when you get into view you should be fully able to fire and the enemy wonít have a chance to react. If you are at very close range, anything, including a medic with a heavy, will die almost instantly.
Find Your Happy Place
On defense, find a place that you can easily retreat to a resupply area, preferably one with a full health and ammo pack. This can allow you to hold an area without the supporting assistance of a medic or an engineerís dispenser.
You + Dispenser = Sentry Gun
Have an engineer place a dispenser for you somewhere and sit on it, literally, just sit on it, and hold down the fire button. The dispenser never runs out of ammo and will keep you healed. This works best as a spy defense on maps like dustbowl, sit right next to the sentry gun and the steam of fire will reveal any spies attempting to sap the SG.
A good heavy make for an impenetrable defense and a steamrolling offense. No team should ever be without a heavy in some role, be it offensive or defensive. Just make sure to always be watching your back for spies, and know when to go and resupply because your slow speed makes timing critical.
Check out the other class guides at the links below
General Tips and Tricks Scout Soldier Pyro Demoman Engineer Medic Sniper Spy