I've kept this one for the lead in to Halloween! Susan Morris provides tips for adding scares to your writing:
Nearly every fantasy story I have ever read has at least one moment where the author tries to scare the pants off me. Often I’m supposed to fear the villain. Sometimes I’m supposed to fear his minions as well. And other times, I’m just supposed to fear a horrific situation, or the probable consequences of a character’s questionable choices. But any way you cut it, knowing how to scare your reader is an important skill—and one whose lack would undermine your entire novel.
She breaks her tips down into:
- Anticipating the Threat
- Perceiving the Threat
- Suffering the Threat
- Stay in Your Character’s Head
There are some good examples in there (facehuggers, yay!), so get the detailed story here: http://www.omnivoracious.com/2012/10/making-monsters-scary.html