Lust's combatant, chastity, is further-reaching than lust; it extends beyond the sexual temptation to help temper and maintain our day-to-day lives. Chastity, similar to temperance, is a practice of a holy self-control.
Specifically for lust, chastity enables us to use discretion in sexual acts and decisions. We've already established that lust itself is not the sin, it is the self-seeking lust that makes it sinful. Thus, chastity is basically avoiding self-indulgence through sexual gratification. That seems simple enough, right?
But what is chastity in day-to-day life? Chastity is respecting our own bodies and the bodies of others. It is not taking advantage of another sexually or otherwise (emotionally, etc.). It is a deliberate, honest lifestyle that is open with those around us. Chastity requires us to reserve sex for marital intimacy. It also demands that we respect the marital intimacy of others. This particular virtue calls for different practice for different vocations, making someone who has been called to Holy Orders called to a different kind of chastity than someone who is called to the single life.
But today we can practice chastity by deciding to not share that 50000 Shades of Grayness trailer. By not making our partners feel ]less by comparing them to our celebrity crushes. By making those around us feel valued and respected for more than what their bodies offer us.