Excellent Spy Awesomeness Technique: Silent Cloak and sabatoge a dispenser. Wait for someone to show up and try to use it. Blow it up. KILL with absolutely NO loss of cloaking or disguise. Even better, that damn Engi will show up again to rebuild. Backstab as soon as he's done, and now it's YOUR dispenser
. Enjoy.
EDIT:
On FF_Aardvark, when you jump off the platform, go to the LEFT and push back on the wall. You will NOT lose your cloak if you do this, helping to not get sniped to hell and back. Again: Cloak in respawn, jump sideways, and hold backwards to slide down the wall. No loss of cloak = More awesome spyness. Get into the enemy base, uncloak, and wreak havoc.