Guard - Guard is a very solid cooldown that will absorb a certain amount of damage as well as increase the effects of your self heals by a certain percentage for 30 seconds, or until damage taken exceeds the absorb amount. The effectiveness of Guard can be modified in two ways.
Firstly, via mastery, which will increase both the amount of damage absorbed and the effectiveness of your self heals, and secondly via Brewmaster Training, which causes Tiger Palm to generate a buff which increases the amount of damage absorbed by Guard by 15%. Since Guard has a duration of 30 seconds and a cooldown of 30 seconds, you have plenty of time to use Tiger Palm at least once so as to maximize the effectiveness of your shield. This ability should be used on cooldown unless you know that a big hit is coming, in which case, you should save it for that hit.