Shield swipe is good for this, as is Flash (i assume you ARE using flash from time to time anyway to apply the Blind Debuff if not for actual threat).

The way TP works, if you stick to JUST a flat Fast->Savage->Halone combo you can run for ages without running out, but it'll gradually tick down. However, if you start using Shield Lobs when the boss is at range to get threat, you'll really run dry faster. It's all about the Average TP per GCD, and when it's below about 50-60 you start gaining it. Shield Swipe and Flash thrown into your rotation help immensely as long as you arent behind on threat.