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The best advice I can give abut playing spy is to think about how you look to others when moving around in disguise. Remove yourself from your perspective and think of yourself in the third person of someone else.

I played spy yesterday too for a round and couldn't backstab a single person. It's soooo hard. Also I don't have a Dead Ringer anymore.

Being a spy is hard, and a lot of TF2 is you dealing with the hand you're given, whether it be team compositions (you or your opponent's) or whatever, especially as a spy. It takes a lot of practice and if you play with some of the elite TF2 players it can be impossible to learn. Just keep practicin'.

That said, my advice would be similar to Kegan's and to try to notice how people look around-where they go, what they do and plan your path of attack accordingly. You can't blend in too well (since you can't shoot, switch weapons, reload or taunt, plus you bump into others) so make your time being seen by opponents as short as possible and don't act suspicious. Only use your gun if you're sure of getting a kill or sure that you've been found out (are being fired at) and be sure to decloak in hidden areas (all cloaks make some noise upon decloaking-even if it's not obvious to you, I've been alerted to an appearing spy by their cloak noise several times) as well as cloak when you're far enough away to not be killed (still, change your disguise, as they'll be sure to be saying "spy pyro!") or where you can't be seen.

I wouldn't recommend disguising as:

-Scout (other players will notice you're not as fast as a scout, nor can you double jump, but it's not a bad disguise if you're running around closer to YOUR base, where they think a scout of theirs might be)
-Soldier (too slow. Spies need speed to catch up to and backstab enemies)
-Heavy (waaaaay too slow and also too big)
-Spy (if you're a spy spy, you are wearing a mask, always. You're usually near their base where a spy on their team might not be wearing one and spies in general make their team think about and more wary to spies.)

That said, I recommend switching your disguises often if you can, whenever cloaked or between lives. Try to act like the class you're disguised as.

Eddie "Skate" Hunter is the twinner of Hoops Barkley.

Quite a few people, I think.

You can actually make it look like you change weapons as a spy.

Press B and it'll change to the weapon in the corresponding slot like if you choose the knife and press B it'll swap your disguises weapon to the melee slot.

my boss is listening to gagnam style

Aaaaactually, if you're doing it right, you won't even give them a chance to figure you out. The only people you have to constantly switch your game up on is engineers, and only if they have half a brain.