Biggest things that made me way better at spy:
Never hug a corner. Always be in the middle of a path and treat the game like audiosurf ninja.
Use the revolver more often than you think necessary (i.e. going for a knife kill on an unaware, but moving sniper who is at 50 hp. just shoot him and cloak -> redisguise -> go again).
Wait for 2 more people to pass after you see a target.
Every time you disguise, use a different class.
Sapping and shooting the sentry is really effective and unexpected. Generally, one dies during/after choosing to do this, but the trade of resources is very much in your favor.
The crazy thing is the gun mettle update made spy way easier for people who play him like I do. I always like using characters in a versatile, utilitarian way. With the resistance changes, to the speed boost options, the spy traded the old dead ringer and gained a new, universal cloaking mechanic. Even so, the dead ringer is still rather legitimate as a watch. One just needs to adapt.
Basically, as cloak is such an important aspect of the Spy, it really benefited from the Gun mettle update.
