Stab then sap, it's more effective.
Also, cooperate with your team and other spies and get them to rush once sentries/etc are disabled so they they can finish them off.
Stand on people's heads whenever possible.
Uncloak during chaos and while moving, you're less likely to be discovered.
Block ubers by crouching cloaked in front of them.
Run in front of a nearly dead medic with ubercharge and await your free spy uber.
Try not to dodge non-crit rockets that are fired directly at you before you've been revealed it's a dead giveaway, unless you're almost dead that is.
Don't move in an offensive direction that the enemy would be likely moving in, you appear with half heath and it's suspicious especially if you just seemingly came out of the spawn.
Go after enemy teleporter entrances, it's cheap points and it slows down the enemy's movements.
Sacrifice yourself to take down dangerous targets.
If you're going to die take somebody or something with you, or pull of a taunt before you're toasted :v
And lastly if you're pointing at the back it will count as a backstab, even if the person is along a corner, this also works from on top of their heads.
