As a Spy, use your Knife to backstab enemies from behind, killing them instantly.
As a Spy, disguise yourself as an enemy with your Disguise Kit. Beware, attacking will remove your disguise.
As a Spy, hit MOUSE2 to Cloak and become fully invisible for a short period of time.
As a Spy, use your Cloak to get behind enemy lines, and your disguise to move around amongst them.
As a Spy, try to act like an enemy while disguised. Observe where enemy team members are, and disguise as one of them.
As a Spy, place your Electro Sappers on enemy Sentry Guns in order to destroy them. Note that disguises aren't lost when placing Sappers.
As a Spy, your Electro Sappers disable Sentry Guns before destroying them. Sap a Sentry Gun before attacking the Engineer.
As a Spy, call for enemy Medics by hitting E while disguised.
As a Spy, be careful when using voice commands while disguised. The enemy team will see them said in the text chat by whomever you're disguised as.
As a Spy, the Ambassador does not inflict critical headshots when cooling down. Make each shot precise and timed in order to inflict maximum damage.
As a Spy, try not to be hit by flames when arming the Dead Ringer, or else the flames may hit you again and reveal your location.
As a Spy cloaked with the Dead Ringer, your silhouette won't appear when colliding with enemies.
As a Spy, pick up ammo and fallen weapons to recharge your Cloak when using the default Invisibility Watch or the Dead Ringer.
As a Spy, the Cloak and Dagger will only drain if you are moving. Stand still or uncloak in order to regain lost charge.
As a Spy, your silhouette can be seen if you move around while cloaked with the Cloak and Dagger for too long. Find a safe spot to sit and recharge.
As a Spy, the Dead Ringer makes a very loud noise when uncloaking. Find a safe place away from enemies to uncloak.
As a Spy, disguise as your own team by hitting the - key. Use this in friendly areas or with the Dead Ringer in order to hide your presence from the enemy.
As a Spy, hit B while already disguised to change the weapon your disguise is holding.
As a Spy, hit B in order to automatically assume the last disguise you previously used.
As a Spy, you can take enemy Teleporters while disguised. Surprise!
As a Spy, bumping into enemies while cloaked makes you slightly visible to everyone.
As a Spy, if you're set on fire while cloaked, the enemy can see you!
As a Spy, use your Revolver to pick off targets that are low on health, or to deal with classes that are dangerous to get near, such as Pyros.
As a Spy, if you are quick, you can stab an Engineer and then sap his Sentry Gun before it turns around and shoots you.
As a Spy, the Dead Ringer significantly reduces the amount of damage taken from all attacks while you are invisible.
As a Spy, avoid taking fall damage as it will give away your location!
As a Spy, your Electro Sappers sap both ends of a Teleporter. Try sapping the end that the Engineer isn't guarding.
As a Spy, your Dead Ringer can be used to fake your own death. Try to use it when you're weak, otherwise it may be wasted or too obvious.
As a Spy, reloading your Revolver will mimic the reload action of the weapon carried by your current disguise.
As a Spy, if you're too slow to sap a Sentry Gun after backstabbing an Engineer, you can Cloak or hide behind his Dispenser.
As a Spy, Your Eternal Reward automatically disguises you as the person you most recently backstabbed. Be aware that you will lose your disguise upon death or when attacking, however.
As a Spy, you can see enemies' health. Use this information to target weakened enemies with your Revolver.
As a Spy, the L'Etranger adds to your Cloak's battery when you successfully hit an enemy.
As a Spy, your Electro Sappers disable Sentry Guns before destroying them. Communicate with your team, and sap a Sentry Gun just before your teammates attack. This will prevent the Sentry from firing, and also make it more difficult for an Engineer to repair it.
As a Spy, the Conniver's Kunai allows you to absorb the current health of your victim upon a successful backstab. Be wary however, as your base health will be drastically reduced when using this weapon, so be sure to remain undetected!
As a Spy, a good technique is to backstab enemies that are alone or are otherwise vulnerable. Always check your surroundings before backstabbing an enemy in order to ensure that you remain undetected.
As a Spy, you can activate your Dead Ringer with fall damage. Feign a careless death!
As a Spy, Bleeding, Jarate, Mad Milk, and emerging from water will reveal you while you're cloaked.
As a Spy, the Diamondback inflicts less damage, but will store a guaranteed Critical Hit for every building destroyed by your Electro Sappers. Destroy an Engineer's buildings before confronting him, and you will have a significant advantage!
As a Spy, taking any fire damage when using the Spy-cicle will melt the weapon, requiring you to wait before you can use it again.

The Red Tape Recorder Exploit was fixed, the one where it makes sentries invincible. So why is it still banned on the servers?