Some cultures cast pinches of salt across cooking pots of food. This was supposed to 'blind the hex' that evil magician types would try to cast on the food.
Salt was also very valuable/expensive in earlier times, because it was much harder to produce and transport. Most people have heard that Roman soldiers were paid with salt. Because salt was so valuable, spilling or wasting it was considered unlucky, and could be seen as an omen of lost income/wealth. Throwing a pinch of the spilled salt over one's left shoulder was supposed to 'blind the demon' of bad luck.
Habits that I still do: knock on wood when any statement I make could be considered to be boasting about good fortune.
This one supposedly comes from Celtic culture. Supposedly, it was believed that the gods/spirits did not approve of boasting. So, people 'knocked on' trees, and later wood, as a way of honoring the gods/spirits, which were seen as inhabiting trees, and nature generally. I guess the gesture was intended as a sign of respect.
Another habit: I don't open umbrellas indoors. My grandma had a cow about me doing that once. She took the umbrella away from me, closed it firmly, and made me stand on her porch in the pouring rain before she would give the umbrella back to me.
Somewhere I read, that belief also comes from Rome, or thereabouts. Apparently it relates to the umbrellas other function: as a parasol. The 'sol' part refers to the sun. It was thought to be an insult to the sun god to open a parasol indoors.
And yeah, I learned the superstition about fortune cookies back in the 1970's, from my former husband. That one is not ancient...fortune cookies began in this country, as an advertisement for a Chinese restaurant. Can't remember if they were invented in the 1940's or 1950's.
Oh, one last one: I always store brooms with the bristles pointing up and the handle down. I also learned this one from my grandma. She said it was bad luck to store brooms bristle-side down...and the bristles last longer and stay in better shape.