I am currently preparing a Call of Cthulhu adventure, and I am hoping to be able to play the game on a regular basis in the future. But I am having concerns about keeping the mystery in the game from one separate adventure to another.
The problem essentially is that even though character knowledge of the Cthulhu mythos does not carry on from one character to another, player knowledge does. I am concerned that the lore of the game will become after some time too familiar to the players, and lose its mystery and appeal. Of course, it is always possible to play adventures that do not use the mythos, and I will do so at times; but the mythos remains, in my opinion, one of the most interesting elements of the game, and I am unsure of how I can keep on using it without wearing it out.
I had faced the same problem when reading HPL's literature; even though the stories were always a pleasure to read, after some time new ones lost a bit of their mystery because I had become too familiar with the universe they were set in.
It also doesn't help that at least two of my players have read HPL, and at least one of them is very well versed in his works.
So how can I keep the Cthulhu mythos fresh for players who already know it well? How do I make old material seem new, surprising, mysterious?