One nice way to keep the momentum going is to provide ways to encourage users to tell others about your product. I don't mean just a "Share" button either.
Right now, you have a form of "virality" if a user does not fulfill his/her goals. But presumably, if users use your app well, they'll never have their embarrassing photo shared, and thus, the success of your app prevents your app from being shared. So are there ways to encourage users to want to promote your app?
For example:
I'm going to take a step back and think about what Aherk is really doing. Aherk is a useful tool using negative reinforcement to encourage goal achievement.
Ah, goal achievement. Is that something an individual would share with others? Perhaps. Some people like to brag about the goals they've achieved. Perhaps you can give users awards or badges for goals they've successfully done, then allow those awards to be shared on Facebook, Twitter, their blogs, etc. Ego gratification can be a good motivator too (albeit in a positive reinforcement kind of way, which is ironically diametric to your current system).
Also, what if a happy user of yours knows a friend who could use Aherk as well? Instead of just emailing your URL to the friend, what if there was a way to post Aherk to that friend's Facebook page or something? This could be accompanied by a playful taunt that the user could type in, such as, "I'll bet you're too chicken to Aherk yourself if you don't finish your paper by next week!" (This feature could potentially be abused, so if you consider this one, think it through some more.)
Hope this helps. Good luck!