I will say.. here is a few tips I have picked up over a long time of playing TFC and now playing FF as well.
General Tips-
1. As a Spy, when entering a base, don't block the enemy leaving the base if you have no intention on killing them, due to the fact people get impatient, out of the annoyance of being blocked, they will shoot at you and then by luck will figure out you are in fact a Spy.
2. Do not spam Gas Grenades in the enemy base unless they already know you are there. Spamming those grenades around will raise a flag for any smart player, especially on a map like Well, they will just go up to the Attic looking for you.
3. If you happen to get into any enemy spawn and are hiding at the exit door, waiting for it to be opened (counting the fact you are out of a Spawn Protector Gun's range and you are disguised and waiting to piggy back out of there), do yourself a favor. Cloak. Anyone will wonder why the door has not opened for you if you are the same color as them. Watch who you piggy back though, getting behind a Spy or a Scout is not good, they might randomly turn around and touch you. Medics are not good either because they may try to heal you.
4. When sabotaging the enemies Sentry Guns, especially if you only have sabotaged one Sentry Gun, make sure there are no HW's near by, if they are near, kill them before, or right after you initialize the sabotage. I figured out the hard way that this: Lvl 3 SG < HW Guy
5. I have seen this with all the novice Spies on the pubs I have seen. Cloaking everywhere you go is not a good idea. When you are moving while Cloaked, you are obvious. You stand a better chance disguised as a bunny hopping HW on speed with his crowbar out. Also, if you cloak everywhere, the other team will start to look for you cloaked, so it will just make it harder on yourself to get inside.
I can't think of any more tips right now, I am too tired, but I will post some more later tomorrow
