But I agree that the Seinfeld hack's greatest strength is also its fatal flaw: Once you do break the chain, all the motivation it provided bursts like a bubble. You've got to somehow motivate yourself to build up another long chain to not break. Until then you're on a "one more day won't matter" slippery slope of sloth.

My startup -- http://beeminder.com -- is trying to get the best of both worlds: stay on a "yellow brick road" to your goal every day. You have to maintain a certain overall average every day but you can build up a safety buffer that allows you to, say, take the weekend off without relinquishing that psychological momentum.