I live my life under the constraint of so many interlocking goals and systems that I can't even keep track of them. For instance, every day I decide how many hours I am going to try to write on the next day (for today, the goal was 3). I also try to write at least 15 hours a week and at least 60 hours a month. In addition, I try to write something on every single day. And finally (this is my oldest writing goal), I try to write 5,000 words a week.
Some of these are easy-to-hit targets. For instance, it's been 970 days or so since I've had a day where I wrote nothing. And I've only failed to hit the 5,000 word target for maybe 4 weeks in the last two years. But some of these targets are a bit more of a stretch. 15 hours a week is more difficult. And sometimes hitting my daily target can be even more difficult, since it's the least flexible goal. Sometimes I just don't have enough hours in a day, and it's not possible to meet that target. And that's as it should be.
In law, there is the concept of efficient breach. Sometimes the damages that would be caused by breaking a contract are less than the cost of carrying it out. In these cases, companies should be allowed to pay the damage and break the contract. That's why contracts should be written in such a way that damages for breaching the contract are not excessively punitive--if the damages are too high, then the company will be locked into the contract even though it doesn't serve the overall public good.
In the same way, a person should design his own personal incentive systems in such a way that the cost of failure is not ruinous, because there are days when it's just not worth it to hack out three hours of writing time, and when you'd be better off doing something else with that time.
However, that's also a dangerous road, because the natural human tendency is to decide, "Oh, today's just not a writing day" and to knock off work.
I think I've found a good balance, in that I don't meet my daily goal on only about 8% of all my writing days. And when I don't meet it, I feel bad enough that I do my best to meet it the next day. But when I don't meet it, I always feel like that's absolutely the right thing for me to do on that day.
For instance, yesterday, it was absolutely the right thing for me to watch five episodes of House of Cards instead of writing for another five hours =]