(Cross-posted to Jennifer's History and Stuff.
Salt and garlic have long been used throughout the world for medicinal and preservative purposes, and as a result generally have "lucky" reputations.
For example: if you spill salt, take a pinch and throw it over your left shoulder to avoid bad luck. The earliest record of this practice dates from around 1584; it was seen quite commonly thereafter. Why do people do this? The reasoning is largely unknown. Some people say the Devil whispers in your left ear and the salt will blind him or drive him away. (As for why spilling salt is unlucky in the first place, this is due to salt's historically high value.)
An older salty superstition said that keeping a bag of salt with a baby before baptism guarded the child against witches. This practice arose at least partly because salt was mentioned in the Bible for use in the baptismal ceremony, and was used in pagan ceremonies long before that.
Garlic also has a history of use as a protectant. The protection against vampires sprang from literature. (See: Dracula, 1897.) Before this, however, it was used to protect against witchcraft everywhere from Europe to Asia.
And since we're talking about modern-day monsters, how about silver bullets? Long before they stopped vampires and werewolves, they were believed to be the only thing that could harm a witch who had taken the form of a rabbit or other animal. Silver itself has a long history of being used for luck, because of its value in general. It is supposedly able to withstand enchantment, which is why it can't be deflected by evil beings.
Reference: Most of the material from this post was found in David Pickering's Dictionary of Superstitions and Steve Roud's The Penguin Guide to the Superstitions of Britain and Ireland.