CoC play by post
I am running a PBP COC game right now myself, though it just began Monday. It takes place in 1930.
My goal for making things creepy is to first establishing a sort of "realism" because most of my players and myself are used to high-fantasy D&D. I created pregenerated characters for the PCs and wrote a brief bio for each which reflect "regular Joes" and their motivation is real too: money or curiosity/knowledge.
CoC Characters are also much more fragile than D&D characters, so maybe give the PCs a little taste of danger; I am having some thugs threaten the PCs early in the adventure. It is a non-supernatural threat but may do well to exemplify the fragility of the PCs; well then again maybe they can bluff their way out.
When something unusual comes into play, I think to mimic movies might be good. Consider some movies hint at horror without showing it, other show a great amount of detail instead. I want to use both. Put characters in situations that erupt in creapiness but aren't necessarily encouters with zombies. Maybe they are taking a shower at an old chateau when blood comes shooting out of the showerhead! Or instead, black sooty water covers them, the shower head brakes off and dirt covers their eyes while at the same moment hands grab at them! Maybe it's a ghost, maybe it's someone trying to help.
Then, when supernatural creatures are encountered, you can give subtle clues before they are "onscreen." Shambling sounds, lumbering footfalls, rattling chains. Finally, when depicting something gruesome or bizarre, use colorful language or detail.
I think one thing you want to convery in CoC that Lovecraft and the other writers convey is a sense of the overwhelming; the PCs may be able to combat or slow unknowable Gods for a time, but really they are powerless against their awesome aspects. When a Great Old One or Outer God is influencing things, think BIG for their impact, like in the film 2001 when you see all these scientists looking at the monolith on the moon. What does it mean? What is it??? It is scary and exciting at the same time; well knowledge of a Cthulhu god is for a moment mind-opening and forever mind-shattering.
Here is a link to my game's website:
http://www.geocities.com/mortalcoil2000/coc_main.html
BTW this is my 2nd CoC game ever and my first for Play by Post, so keep that in mind; I am representing my approach to things in this post.