Some tricks.
1. When you give slips, give everyone one as already mentioned above.
2. When you give only one PC slips, don't just do it when you need to; You can just give random stuffs or red herrings as well at random intervals.
3. Tell players to "no meta" as already mentioned above.
4. Randomly roll dice and make notes of them, even if those are useless rolls.
5. Prepare a "Route B" that you'll only used if most everyone seems to have meta-ed the whole thing already.
(Think of it this way: If you ever need to pull Route-B, you've already failed as a GM, and its only for salvaging a game that's pretty much proven dead. Prep it, and try as hard as possible to not use it.)
6. Try to keep this a one-night only thing. Longer someone have time to think, more holes and less shock there are.
7. Be lenient on the mechanics: You don't want a game to be f__ked up only because someone f__ked up one single roll of dice.
7b. Make sure that vast majority of your clues aren't dice dependent, to avoid #7. (Reason why GUMSHOE was created was exactly due to #7.)
Or, if you're gonna run CoC like GUMSHOE, just run ToC instead of CoC.
