Just, just, just. Well, you just need to take this wire brush and cram it all the way into your asshole.
My favorite example -- because it's a conversation happening ten thousand times on this planet as we speak -- is when any obese person talks about how hard it is to lose weight, and their thin friends helpfully tell them they "just" need to watch what they eat, and "just" get a little exercise. They "just" need to win the brutal war that has exhausted every ounce of their energy, time, and emotional well-being for as long as they can remember. "And to be honest, dude, it's kind of weird that you and the other 1.4 billion obese people on Earth haven't tried that already."
I'm sure I've said this to people many times over the years, along with all of the other terrible advice on this list. So if I could go back in time, I'd tell Past David, and anyone else offering this condescending, dismissive, wet shart of an attempt at advice, to please grasp something:
The fat people you make fun of and condescend to probably have more willpower than you do.
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And they look much more awesome in a headband.