First of all, start with a different game (system.) Using anything Lovecraftian (CoC, ToC) would be a dead giveaway. Pick a (very) easy, generic system, preferably something your players are not familiar with. You'll want to tell them you read an interesting review about it someplace and would want to give it a few shots, to freshen up your gaming.
Previous to announcing this, watch (or just pick an already seen) movie or TV series (or computer game) that deals with the 1920s era (roughly) and that you're familiar with - and that has no obvious link to the Mythos. (HBO's Boardwalk Empire or LA Noire would be good choices these days.)
Now, tell your players about both the interesting new indie game and the movie/game you're so enthralled with nowadays. Prepare them for a story set in the vein of the movie/game. (Go for crime and investigation, that's an easy one indeed. Do borrow from your source.) Never refer to Lovecraft or Cthulhu. If your players bring them up, try not to confirm their suspicion.
Have your first few stories be really not about the Mythos. On the contrary: use red herrings. Throw a rare few Lovecraftian adjectives and descriptions about NPCs ("smells constantly of fish", "slightly protruding eyes" etc), use a few conspicuous props (strange paintings, a suspicious house) - but always present logical explanations for these. (The fish-smelling guy really does work as a fish-seller, the eyes are a medical condition explained away by a doctor whom the party trusts, the paintings are the works of a poor drug-addict artist, the house is architecturally unstable and is demolished during the story etc.)
Then, after two or three short stories (but still using the new system), start revealing the true nature of your campaign. Switch to ToC only when you've had the first real Mythos encounter/event.