With The Shining, it was all about crazy angles and interesting use of colors and patterns. It makes things....trippy and unnerving to look at.
First off, thank you! ^_^ I'm not sure I've perfected it, I just try to work with what I find the most enjoyable, so it makes it more fun. In Daniel's case, it's what scares me the most about vampires. The idea of having a loved one die and return as a horrifying thing that lacks feeling, but still has your friend's looks and some parts of his old personality, that idea I find psychologically disturbing.
Drawing what you find scary is a good idea, naturally. If it doesn't disturb you to an extent than chances are it's not going to bother anyone else much, or you'll get bored fast with it if it's not having an effect on you. XP
I've just watched a TON of horror movies and am super desensitized as a result, so it's easier to draw the disturbing things. XD There is mainly just one scary character in my comic (Daniel, of course) and I'm not so much afraid of drawing/writing him (though I've creeped myself out a few times), though I certainly would be scared if I confronted him in real life. o_O
With threatening atmosphere, dramatic angles are your friend, and if you go the darker route with the look, so are shadows. Also a character's expressions and body language can make a world of difference in how a scene is supposed to come across. If a character is losing their mind, you should see it through their expressions, their body language...and sometimes a more subtle hint of insanity shining through can be more effective than a BIG TOOTHY slasher grin.
Slow build-up and not knowing exactly what's happening can also be effective with horror writing. The unknown can be a very scary thing.
Aaaand now I'm rambling, never get me started on horror writing. XD It doesn't help I'm bad at trying to explain my thoughts on these things.
Horror is one of my favorite genres because it comes in many different 'flavors', the best advice I can give is look at a whole bunch of horror stuffs, pick and choose the ones that had the most effect on you and try to learn from them. What's the main theme? What's the atmosphere like? What bits creeped you out (or just entertained you) and why?