Nightmares, Night terrors, Bad Vibes and the Things that go Bump in the Night.
This shall serve as a reference for keeping such things at bay.

(Do keep in mind however that an occasional bad dream might just be to teach you something).

help with nightmares
Horrible dreams (possibly a spirit)
Frequent Nightmares
Nightmares and a New Home

Some references from elsewhere on the MysticWicks site,
Magical Properties of Incense, there are several that are useful for cleansing a space of negative influences, preventing nightmares and protection.
Symbols for nightmares includes runes and charms to prevent them.
Protection - A thread by someone that felt magically targeted and reader solutions to this problem.
Dreams and Crystals - suggestions of stones to keep bad dreams away.