Kill the PCs as fast as possible. Their mental blood and guts will grease the imaginary wheels of your CoC adventures. In the alternative, sweat, caused by fear, terror, horror, and unusually high heat can achive the same result. Horror sweat is the best.
The best of both worlds is to kill your PCs after they have become extremely sweaty. The best way to get both of these things is by allowing the PCs to destory themselves. Know the world, know the people in it, and allow the PCs to blunder through, sealing their own fates with their "fool proof" plans and schemes.