One concept that I suggested in an older thread has had some success. It's a little more work for the GM, but can have good results:
Don't let the players track their own Sanity scores or Cthulhu Mythos skills. Make a note of these at the start of play for each of them and track any changes that occur during play. When a character loses some SAN, have them faint, or run screaming or whatever, then afterwards, when they've developed a disorder; start "gaslighting" the party. Examples:
A) Hallucinations. Ask the players what their Spot skills are and roll a bunch of meaningless rolls. Then tell the player of the "crazy" character that they just saw something move in the shadows. Everyone scrambles for guns & flashlghts...Nothing there. Offer no explanantion as GM, you're just telling players what their characters see and hear. Other good examples of this are Listen checks to hear "Rats in the Walls", noticing bloodstains, etc.
B) Ghostly grabbing. Wait until the party is in a spooky "haunted house" kind of situation: dark corners, nerves on edge, they just
know there's cultists or zombies or something! Then, ask for marching order and you pass out notes to the whole party. Some of them say things like "You seem to remember reading something about a clock like the one on the mantle, but you can't recall where offhand.", etc. but the "nutjob" gets a note saying "You feel a clammy hand grab you roughly by the shoulder from behind!" The character whips around screaming ready to shoot and all the other players don't know what's going on. Of course, nothing's there.

This works especially well in a situation where the party is attempting stealth to avoid detection.
C) Paranoid delusions. The "Crazy" character
knows that a certain NPC is lying to them or wishes them harm. This works especially well with a PC who has any ranks in Sense Motive. Pretend to roll for them and let them wonder whether they succeeded.
The first two are a lot of fun and eventually, of course, the party begins to think that the one character has just "lost it." Of course, that's when the "Crazy" really
DOES see a ghoul leaping over the banister to attack them!
One other thing I would mention is that -unlike a Fantasy RPG- Horror roleplaying requires a certain amount of cooperation on the players' parts. I mean, part of the POINT is for it to be scary! It's easy to wreck the mood in a horror game. There is some good advice in the CoC d20 book on GM-ing horror RPG, too. One thing I find useful is to instill a sense of urgency in the players. They can't afford to take weeks off for therapy or healing; the whole bloody world is going to end at midnight tomorrow unless they stop the cultists, etc.!
Hope this helps.