For a long time, I have had an em-dash habit. I use them a lot–perhaps too much–when I need to make a parenthetical or emphasize a clause. My theory is that since I gesticulate when speaking–hands and face, with funny voices–I feel the need to make certain phrases jump out–as if animated.
Recently, though, I’ve taken up with the semi-colon. I tutor several AP English students and make them do the editing exercises at the end of The Elements of Style. It’s then, after I beat them about the shins with far, far, too many commas, that the chimera mark starts cropping up. It’s insidious; there’s little I can do but wait it out.