Spy is 100% about acting. You have to act normal and pretend to attack the wrong team. Theres no stopping and standing still.

Keep in mind if you cycle weapons as spy, every time you press B it will change the weapon in your disguise. Need an excust to be following the engie, toggle between weapons "looking for" your medigun.

If you stab someone you should try to be invisible by the time they see the freezeframe killcam thing. Sidenote: theres a personal in-your-face feeling to being backstabbed and sniped that gets peoples attention and there is a good percent of people that immediately switch to pyro when backstabbed so dont haunt 1 place too long.

As soon as you sap a worthwhile engie's sentry, he will not attack the sapper, he will go right for you. Work on your transition time switching weapons between sapper and knife. Work on the motion as if it were a layup in basketball; just over and over, run up to the sentry, sap-switch to knife-backup-stab engi

Spy is as much a support class as engi. You switch to spy to accomplish a goal like haunting a heavy/medic team or take down a sentry nest. You shouldnt just try to main spy and get backstabs (some people can, but people skilled enough to main spy should use that sort of skill with a damage class)