Team Fortress 2 - Spy Guide
Like James Bond, but Frencher and Stabbier
Live and Let Die
Dressed to Kill
To maximize effectiveness, a team should coordinate with their spy to sap sentries at the right time. It can also help for a spy to backstab key players at the right time in order to help open a hole for an advance. The biggest challenge that a spy faces is remaining unidentified by the other team. You should only unstealth in disguise when you are completely obscured from the opposing team. Try to only pick disguises that will be believable to the other team. Snipers often work well if you're in the back of the map, as do classes like the soldier. The best place to decloak is in the water - it's very hard to catch someone cloaking or decloaking while they're underwater.
Often, you can get multiple backstabs in a row when you are going after a medic healing someone in front of them. When you see an opportunity to make a chain of stabs, it's often a good idea to go for it. Don't be shy about informing your team over voice that you've just slaughtered a bunch of people with little resistance! It might be your chance to basically win the game for your team.
If you find yourself getting slaughtered constantly by the other team, it's a sign that you should change both your disguise and your approach. You want to keep your actions as unpredictable as possible so that the other team doesn't have an easy time of picking you out of the mass. Don't play a class that the other team has only one of, or they'll be able to tell that it's you immediately from your false name. Try to keep your face pointed towards your own team's side, so it's not as obvious that you're going after the opposing team's defenses. Also, don't go for every backstab opportunity that opens itself up to you. Keep the other team skittish and unsure when you might next strike, and you'll have a stronger psychological effect on them.