When you're sick and your nose and head are feeling congested, the first thing you'll try to do is blow your nose as hard as you can to dislodge all of the nastiness lurking inside. That's what you're supposed to do, right?
As it turns out, blowing your nose can actually push mucus farther up inside your nose, causing your sickness to worsen or last longer. That's the opposite of what you want!
Instead, try gently blowing just one nostril at a time, or take decongestants instead.