Well, if you want some tips right here in your own thread, here are some:
1. Don't get frustrated. Truse me, I am probably the king of this. If you can even get to the other teams side, don't go crazy, keep trying, join a different server, or even turn off TF2.
2. Practice seeing how long you can go just blending in with the opposite team without getting killed. (Don't backstab anyone, just walk around and pretend you are contributing) This will vastly improve your skill to blend in, and live longer.
3. Stab a bit before you normally would. Usually, I go to stab too late and they enemy ends up moving. If you stab a bit before, you get the quicker stab animation, and a higher chance of getting the backstab.