==========Jelly says:
Disguising yourself as a Pyro on your own team generally means the enemy will keep their distance, and thus, you can cloak and appear behind them.
You're visible while you cloak, so run in the opposite direction you want to go until your arm turns fully transparent. Then double back and watch everyone shoot the air behind you.
A disguised Spy will not bleed when shot by an enemy unless is kills him, but a Pyro's flame will set him alight and a Demoman's Grenade will detonate if they hit him.
A cloaked Spy reveals himself for about a second if he brushes against an enemy or is shot. He also drips visibly when emerging from water.
The shimmers you see running about are friendly Spies, but their name tags don't always appear.
Definitely not enemy Spies: A Spy wearing a mask, a Medic with an UberCharge percentage, anyone not holding their default weapon and anyone under 50% health.
Changing disguise creates a huge puff of smoke and takes longer than you probably think. Only ever do it when completely cloaked or in a dark corner.
When emerging from a Teleporter, you're trailed by team-colored particles. These take longer than walking the length of the map to wear off, so don't use teleporters.
If you constantly replace them, your Sappers will damage an Engineer's sentry faster than he can repair it. Disguise as an Engineer so his team wont know who to shoot.
Players usually emerge from Teleporters facing the same direction. Instead of destroying the Teleporter, stand behind it and back-stab the players who come through.
When disguised, you appear slightly injured to the enemy. Enemy Medics healing will increase this level, but Dispensers won't. Both, however, restore your actual health.
==========
ßðÐŻÍ££å says:
Changing your disguise while Cloaked lets off a particle effect which will allow your enemies to notice that you have just changed disguises.
When attacking a Sentry Gun guarded by an Engineer you have two ways to take it down and kill the Engineer quickly.
The best and easiest, but slowest method is to place a Sapper on the Sentry Gun first, then Backstab the Engineer before he has a chance to knock it off. This will generally alert the opposing force that a Spy is Sapping the Sentry Gun via scripted voice actions from the Engineer.
The quietest, fastest and most difficult way to do it is to position yourself behind the Engineer and wait for the turret to turn so it is facing the same direction as you and the Engineer. If you're fast you can Backstab the Engineer, switch to your Electro-Sapper and Sap the Sentry Gun just before it has you in its field of fire. You need to have quick keyboard and mouse skills for this, so if you can preform actions such as double melee or backpack reload in Halo PC/CE you should just be fast enough to preform adequately.
==========
Post your own tactics