I spy all the time, and while I'm not the best at it, there are somethings I've noticed myself:
1: A team with more than 4 spies on it makes it harder for offense to capture, defense to defend and other spies to take out priority targets (ie, medic) when there's a fool spy stabbing the razorback sniper. Generally, the less spies, but greater than 0, the better.
2: If you use the DR, get out of the area under cloak quickly enough that you can gain a safe uncloak space. Never wait around to decloak behind someone, especially when not using the Saharan set bonus.
3: Realise what the guns are for: Revolver is for finishing off those who moved when you went to backstab/when you've been found out. Ambassador is for killing anyone slow enough (usually snipers, especially razorback) that you can take a headshot and kill quickly. L'Etranger is for the set bonus or to refill your CnD or DR, do not underestimate this weapon. Enforcer is for when you're around many wounded enemies that you can kill quickly but cannot get a headshot on, also good when running from pyros. Using the YER, DR and Enforcer together no matter what the situation calls for, because it's "cool" is quite foolish.