What do the following have in common?

• "Trying to lose weight? Just eat less and exercise more!"
• "Trying to get more done? Just stop wasting time!"
• "Feeling depressed? Just cheer up!"
• "Want not to get pregnant? Just don't have sex!"

Answer: they would all be quite successful if followed, but they are all difficult enough to follow that people who actually care about results will do better to set different goals that take more account of how human decision-making actually works.

The specifics are very different in each of these examples, as Wes_W noted here.

"Trying to lose weight? Just eat less and exercise more!"

Is the only one that's potentially actively bad advise, for the same reason "pee a lot, don't drink any water, and stay away from heavy food like vegetables" is bad advice.

"Feeling depressed? Just cheer up!"

Is at lest not actively bad advise, but "cheer up" isn't a primitive.

"Want not to get pregnant? Just don't have sex!"

Is actually good advise and a little investigation reveals that the people saying otherwise are placing nearly as much (or even more) value on "you having sex" as on "you not getting pregnant".