Altogether, assassins have 4 defensive cooldowns: Deflection (weapon damage), Force Shroud (tech damage), Overload Saber (lockout-free medpack), and Force Cloak. I use Overload Saber fairly aggressively, and I'm always surprised at assassin/shadow tanks who don't. Force Cloak is another uncommon one, but primarily due to the fact that it debuffs healing. It has to be used very carefully with full knowledge of what you're about to do to your party. I use it absolutely every time I tank NiM Kephess (twice in the same fight if my co-tank dies in the last phase). Similarly, I use it every time on NiM Toth and Zorn (to reset the medpack lockout), fairly frequently on HM Dread Guard, and every time on HM Writhing Horror (to avoid pulling off my co-tank at the start). Also, there's all the situational uses when you need to get someone up and the battle rez is down.
No question though, the bread and butter is Deflection and Force Shroud. Use and abuse the heck out of those cooldowns. A huge part of what separates good assassin tanks from great ones is an active knowledge of the specific fight and the timing of relevant damage (e.g. Force Shroud right before Savage Wounding is applied on NiM Kephess).
Overcharge Sabre not Overload Sabre.