I have a very good strategy and am typically at the top in frags. I use the spy like no one else I know. Here are my tips. I am mostly a defensive spy. (odd u say) just read on. The spy best weapon is his feign and dart. never under estimate the power of the dart! What i do is feign out in the open where a enemy has to walk and change skin. When someone comes unfeign and shoot them with a dart. Then quickly change to nailgun and run circles around them. You can take out ANY player doing this. Typically it is good to do this near the flag. If someone gets it thier not going anywhere fast and if you dont get him your team will. Players will then waste A LOT of time shooting dead bodies. P.S. plus it looks really cool to have someone unfeign and shoot the enemy ; ). -Dont believe me? Ask my favorite([DWG] TOOL)victim.
Submitted by [G.O.D]Link

Well, I havent played spy much, I am usually a Medic, but one thing that I learned when I played as a spy was this: Dont dress up as a medic and hang around one person constantly. They will get suspicious as to why they have not called for a medic and why one is hanging around them alot. Also, dont pull out your knife until you have a GOOD shot at them. If they're moving around alot, you might hit them only once with the knife before they figure out what's going on and waste your ass. Also, DONT HEAD FOR THEIR FLAG. Stay away from it. They usually have an engineer to shoot people entering the flag area to make sure he's not a spy, and if you're discovered, especially in 2fort4, then your ass is grass as every enemy defender within the country will be on you like a pack of dogs on a 3 legged cat! NEVER get near their elevator either. Especially with people hanging around. When you get on it, and it doesnt work, smart enemies are going to see whats going on right away and waste your ass.
Submitted by [MiB] Adrenalin

I dont know how relevant this is to your cause, but it is somthing fun to try, I have done it a couple times and it always makes me feel cool: try to be a spy and not get killed even one time in the game, I dont mean by standing in your lower resupply, I mean go to the enemy base and kill sentries and snipers the whole game and not get killed. it is a rush! (actually, this is my strategy on 2f4!! - SteelAngel)
Submitted by Il Duce[UVM]

I found it helpful to talk to other spys and get together at enemy water and go in together as a medic and hwguy
Submitted by (LIL)Rocket_PCW

Make note of the main enemy classes (which there are most of) and use them as your disguise choices...after EVERY attack change back back to the enemy colour and then choose a new skin....OR, look at the enemy classes and decide which one they seem to need the most...if they haven't got an engineer, change your name, and become one...!
Submitted by [QC] sin

I've always found that, while spying an enemy base, running around as if you were doing something of utmost importance that you mustn't be bothered or else your plan will fail works. Usually they'll see (insert color here) player running into a hall entrance, they won't think much of it. Wait till they see a "knifed message" before switching skins, and then run around to where your new skin class usually hangs.
Submitted by Pepper

In 2fort4 I generally go in as point man for the team and grenade any guns/guards at the top of the ramp leading downstairs. I then (when no-one is watching) feign as an enemy engineer just off to one side while my buddies make the flag run. When the enemy sees the "Your flag has been taken message" there will invariably be one or 2 guys rushing to the top of the ramp to cover the exit. Just as my buddies are rushing up the ramp I leap up and knife the would be ambusher in the back. Spies used as covering fire like this are excellent and it makes perfect use of the feign command. Submitted by Maquis ICBM