I used to eat the play-dough.
I used to eat the glue.
I would write my name on everything (hence why I now do graffiti, bad habit turned to...well good for me, bad for the haters/po-po).
I picked my nose till it bled....eeeeevery time (and still do sometimes)
Not so much a habit, but as a kid, I would cry at the slightest bit of pain. Grew out of that through skating. You can't really bail a trick down a set and then start crying...I mean, I am an image whore.
I used to blow bubbles with my mouth constantly. (I still sometimes do it...at eighteen. haha)