I'm not gonna come out and gloat that I am a great spy, cuz I am not great, I'm just good. Being a good friend of Quantum on 2FORT IR and playing with him for the last bunch of years, you tend to pick things up.
First off, do not use dead ringer....do not become one of those spies who think they are the best cuz they can move around with it. Cloak and Dagger > Dead Ringer.
Before you join a team, take some time in spec, 5 or so minutes, look at the opposite team, notice where the sentries mainly get placed, find habits of some of their players (pyros mainly).
Be patient, you can't just start stabbing the first person you see, patience. I'm sure everything will start coming to you the more you play.