I like using grenades and the nailgun to take out the sniper's on the ledge then feigning in the corner that way they don't know your a spy and i change to their color so they think it's them. Then just wait for a mininte or two......jump up and knife'em, I've done this twice got one of them and feigned before the other one turned around he thought a snipe got him,and when he went to get the pack I got him.
Submitted by [GI]Snake-eyes

When a sniper shouts: "Hey, are there any spies in the vicinity?" All spies (on your team) should declare themselves and their current colors and skins to avoid unnecessary confusion. That way we know who to shoot. (But then again, snipers shouldn't shoot blue people going into the blue base: SteelAngel)
Submitted by an anonymous sniper

The spy can even be used defensively. Is there a scout just running from base to base with your flag and no one can stop him? stand by the flag with your buddies at the flag and when he shows up tranquilize him. That will make it a tad easier for your team to kill him.
Submitted by _FU_CosmicJim

My trick is to wait at the flag and pretend your guarding it and when the guy who's beside it goes to get a reload, you take the flag and haul ass, usually they think you'll defend it
Submitted by Insomniac

If the opposing team have a weak inside defense, in a map like well6, but a strong outer defense of snipers or sniping soldiers etc, Kill all the snipers and it will give your team enough time to attack safely.
Submitted by IceBlue[MoC]

If you died in the enemy fort, your dead body is still lying around there and shows your skin and color! (I noticed this too. And I have been using it when I do my defensive forays. - SteelAngel)
Submitted By ]Stealth-Viper[ Clan CyberKnights

Another good way to succeed as a spy is to act dumb. Rush into their base repeatedly without changing skins. They'll think you're a complete dumbass and ignore you. After a while start acting smart, using multiple skins and feinging a lot. (Sounds dumb right? Actually it isn't as dumb as you think. Psyches out the enemy. - SteelAngel)
Submitted by ?

Feigning death can be a spy's greatest weapon, if someone thinks your a spy and shoots you feign and 9 times ouuta 10 they walk away. or feign up where the snipers hang out, as a dead sniper. Wait untill they are concentrating on the enemy and knife em.
Submitted by =<T3P=

When you feign do it right up against a wall, because usually half you body will go through the wall and reduce your visibility, and don't move while feigning this is a dead giveaway!
Submitted by Isle of Sark

Here's a mistake I see alot of spys make: They run into a guarded flag room grab the flag and get killed by the flag defenders. Get rid of the flag defense first and then grab the flag, otherwise it's a waste of time. And if the flag defense is really heavy, gas the place to the high heavens. Remember that a spy can carry 4 gas and 4 regular grenades. So if there's heavy defense stock up before going over. Also most spy's I see will always run away from a fight. The spy is better armed and armored than the engineer and the scout so if they bust you kill them!
Submitted by Isle of Sark

It isn't very wise to disguise as a medic... the real medics usually fire the medikit as they are walking towards you, and spies that attempt to do this will undisguise. Be a disguised pyro instead, for not many suspect a pyro... Don't waste your time on hitting soldiers and hwguys with the tranquilizers. It does minimal damage, has a long reload, and their tranquilized speed is already their normal speed. Here's another tip, not quite effective, but hilarious: Feign, especially in 2fort4, in the upper resupply entrance. I did that a few times and the enemy was mesmerized! : ) Spies are also awesome for defense - when an enemy gets to the basement, chuck gas grenades to impair their vision, leaving them open for the kill.
Submitted by Crispy Sasquatch