I came across this very nice article on exercise motivation. It points out how the "no pain, no gain" paradigm is essentially flawed as a long-run motivational tool. It also chronicles the success of college football coach John Gagliardi, who uses a very autonomous system of coaching.
Gagliardi's program is essentially hands-off, removing all the external motivational techniques and instead relying on players' natural passion for the game. He uses no whistles, no lap running, no tackling in practice, no mandatory weight-lifting. Instead, the players line up and play touch football during practice, running plays until they are working smoothly. As for the results, Gagliardi is college football's all-time leader in wins and hasn't had a losing season in 39 years.
So what's better for exercise motivation, cracking the whip or stoking the fire within? Obviously, cracking the whip can work in the short-term, but exercise and health are lifetime concerns. It's better to be nice to yourself if you want a long, healthy life.