Not attending to the trash
Taking out the trash is a fairly mindless and effort-free household chore, which is why you have no excuse for those tissues that are toppling from the wastebasket onto the bathroom floor, or for a kitchen garbage can overflowing with grounds-filled coffee filters and egg shells. Better yet, kill two birds with one stone and open the fridge to check for spoiled food right before you take out the trash!
Having bad table manners
Talking with your mouth full. Wiping your greasy hands on your shirt rather than using a napkin. Reaching across other diners to grab the salt and pepper shakers. Those are all actions that relay to the rest of the world that you have bad table manners, and by the time you hit your 30s, you should be able to do better than that.
Not wearing clean underwear and socks every day (this means no holes!)
There's nothing quite like the excruciating embarrassment of turning up at someone's home, being asked to remove your shoes, and realizing that your socks stink and are full of holes. A good way to prevent that is to act like the responsible adult you are and wear a fresh, hole-free pair of socks every day. Fortunately, you're less likely to be asked to remove your pants while visiting a friend's home (or maybe you are likely to be asked to remove your pants, in which case, I'd like to get to know your friends better!) Still, though, the same holds for underwear -- clean and intact is what you want to aim for, every single day.
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Jolie Kerr is a cleaning expert and advice columnist. Follow her on Twitter: @joliekerr.