Hearts can harden fast. The writer of Hebrews drives this point home: "But exhort one another every day, as long as it is called 'today,' that none of you may be hardened by the deceitfulness of sin" (3:13).
So for pastors—and for all of us—yearly or quarterly or perhaps even monthly accountability is dangerously rare. The hardness that creates "an evil, unbelieving heart, leading you to fall away from the living God" (Heb. 3:12) can happen in a day.
John Piper and the other pastors of Bethlehem Baptist Church know this, and so, among their other strategies for sanctification, they hold each other accountable with a simple questionnaire (PDF) that they each fill out weekly. It addresses issues from days off to diet, personal devotions to pornography.
They would all agree that filling out the form is not what matters; what's important is the heart behind it—the desire to be pure and holy.
I think they would also all agree, however, that committing to answer these simple and straightforward questions each week is an invaluable tool in the fight of faith.
Update: A reader offers a challenging reminder about sexual accountability.