I was told once that Clues for the readers should a lot like a woman's dress: Veiled enough to keep the plot covered up, but revealing enough to keep up interest.
How do you go about slowly revealing clues to your readers in your books? If you remember watching "Sixth Sense" the second time around, you probably managed to catch almost all of the clues that the Bruce Willis "Psychologist" character verbalized to you, sprinkled all through the movie. I used that pattern to some good effect in "For All The Marbles". Those who have gone through the manuscript the second time have all said, "Ah, now I see what he was up to. It was there all along!"
In my next book, currently titled "Mandate", the only "clue" that is presented at the outset of the book is a picture, taken from a spy satellite, of the airplane boneyard at Davis-Monthan Air Force Base near Tucson, AZ. And, on the picture, someone has scrawled a single word: "ANTS!!!"
I will be very deep into the book before my detective/investigator will discover through a casual conversation with an intelligence technician that hidden among the countless aircraft in the picture is a small fleet of Antonov-124 Russian transport aircraft, which the technician will explain are commonly called "ANTS" at the NORAD Command Center, where they track all foreign aircraft: Foxbats, Floggers, Badgers, Bears...and, of course, ANTS.
In the story, the previous owner of the picture was murdered. He was an ex-Air Force pilot, and it was his hand that had scribbled ANTS on the picture. When the detective stumbles across the true meaning of the word, he suddenly picks up the trail the victim was on when he was murdered. Of course, now being on to the same information, the detective's life becomes threatened, and we will move toward discovery of just who is behind this murder, and just what ANTS means....why there is a group of ANTonov-124's hidden in plain sight in what is supposed to be a scrapyard?
Have fun with your clues. If you can, put them out there in plain sight and tease your reader. It will keep them turning those pages, one after the other!