These are Spy-specific rules I follow(and made)to get the most out of the Spy.
I'm not sure if there has been anything like this,but what the hell.
Entrance-Find their blind spot.Once you find that it's safe,get in there and disguise.Be sure not to run into Pyros,Demomen,Engineers or any other dangerous classes.find a spot close to where your target(s) are,and strike while their distracted with friendly classes.
Examine-Find where they have Sentries Pyros and the likes,and avoid these at all costs,unless death is your cup of tea.Find a Sentrie,Sap and stab the Engineer.make sure to avoid the Pyros.
Elimination-In my opinion,the Spy's primary objective.Sap Sentries,kill classes deadly to friendly classes,e.g Spies,and take them down as quickly and as quietly as possible.Snipers,Heavies and Spies are all deadly to them,so kill them.If possible,enlist the help of another spy on your team,and you can land a few backstabs together.
Escape-Whether it be running off with the enemie's Intelligence,or yyou've blown your cover,run like hell.Get someone to distract your pursuers,but if that doesn't work, keep running.If you're not running with the enemy intel,run behind a corner,cloak,go behind them and land a backstab,disguiseas who you killed and keep running to give the enemy the appearance that you're chasing after the enemy Spy,especially if thgey're slow,like heavies,and need time to catch up.
If you disguise as a Heavy or Soldier,you can get your health up for a bit,cloak and get behind the Medic and backstab,unless they were expecting that.
Mangled7