Here's some of mine:
Biting your fingernails will give you appendicitus. (That one made me stop biting mine)
Swallowing your gum will cause your bellybutton to stick to your backbone.
Nose itches? You're gonna kiss a fool.
Foot itches? You're going some place you've never been before.
Palm itches? You're coming into some money.
Crossing your eyes will make them stick that way.
If my g-ma ever caught anyone scratching she would say "Ain't no use rollin' 'em over, they got legs on both sides
"
And this isn't really an old wive's tale but it's something my g-ma, who had 15 kids and was very wise always told us to live by:
"Don't believe anything you hear and only half of what you see and you'll get along great."