1. Letting themselves get out of shape. If you’ve ever watched The Biggest Loser and questioned the contestants desire, based on their struggles -- you’ve never been out of shape. You’ll find that the rumors are true, and your metabolism moves slower than a snail at the DMV as you get older. If you continue washing down brownies with Mountain Dew and discounting the value of working out, you will pay.
If you identified with former Glamour blogger Erin Meanley’s blog post "I Need a Do-Over on My 20s!"—to which I offered my own thoughts here—you'll probably appreciate this article too (via Jessica Spiegel @andiamo). And there's also Joy the Baker's popular post "Turning Twentyone." I love those kinds of lists. Update: Here's another list, "What I would Tell My 20-Something Self,” published in The Huffington Post.