Now, before we begin here, I ought to let you know why I am qualified to write this list. I am an Agatha Christie devotee - and have guessed the endings of a good many of her books, which is no mean mental feat, I must say. I have only been truly surprised by the twist ending of one book in my entire life, and it wasn't one of hers.* I have read all but one of Madame Christie's stories. (It's one about Poirot going to the dentist, and finding it is the quest of my life.) I've seen the movies, the TV show, and read commentaries on her. I subscribed to both the UK and US Agatha Christie newsletters, which has proven rather pointless as they're exactly the same. I own the Agatha Christie Who's Who. I once even went to a meeting of the local Murder on the Orient Express book club.
(These are some of my Agatha Christie Books. The hardbacks are not shown.)
So, obviously, I know what I'm talking about here.
My mom once asked my Grandpa, who was a surgeon and knew every answer on Jeopardy (and so was, clearly, a genius), if people really could guess the ends. "You can guess," he said, "but you'll guess wrong."
Now, with all due respect to Grandpa, you can guess right. The likelihood that you'll get every detail right is slim, but you can guess who did the foul murder. I'm not a surgeon and I only know Jeopardy questions that relate to the Beatles and Johannes Gutenberg, so if I can do it you can too.
Basically, Pay attention. Use zee little gray cells!
This, really, is the only thing you have to do. People claim that she doesn't give you all the clues to the mysteries - but Agatha Christie herself declared that was not true. You just can't read an Agatha Christie novel lazily or with your brain turned totally off. If you're serious about guessing, you must pay attention. This manifests itself in a number of ways.
1. Nothing is arbitrary.
This is something all the best mystery people do. You have to assume that everything - every passing remark, or thing noticed or action taken is significant. Everything is connected. These books are like... baclava. Or spider webs. They seem delicate and fluffy, but they're labor intensive and carefully organized. Recognizing that will help you immensely. To use an example from a Dorothy Sayers book, Have His Carcass, putting together what it means that Paul Alexis's joints hurt after activity, and that he refused to shave - seemingly tiny things that are mentioned off-hand - have to do with each other solves the whole mystery.
2. Don't assume you won't be able to guess because there will be historical things you won't get.
I sometimes complain about this with Sherlock Holmes - occasionally the stories seem dependent on facts that, had I lived back in those good old days, I might have known. However, only once - once, mind - in all my readings of Agatha Christie (in Murder on the Orient Express) did I think that there was something I couldn't have gotten because it was a historical fact. And if I was British I wouldn't have missed it. I'm not a history buff, so you can't claim to be fooled by this.
3. NEVER write anyone off.
Poirot himself is forever explaining to Hastings that he suspects everyone until the very last minute. And you ought to do the same! Don't fall into the habit so many Agatha Christie characters do themselves - pinning their money on one character being the murderer, and making all the facts bend to fit the theory. It just doesn't work that way. You have to take every piece of evidence against each person and add it to their list. Then, at the end, you see whose evidence is the most convicting and ba da bing! You have your murderer.
4. Think dirty.
To half-spoil Hercule Poirot's Christmas, when the murdered man boasts of having sons born on "the wrong side of the blanket," (after taking careful note of it, per number 1) think about it. He's got illegitimate sons, eh? Are any of them around? This woman has died and there's a chap there whose half her age. Could he be her son? Use a weird mix of logic and creativity when you think about people's social stations or ages. Never believe people when they say who they are.
5. Enjoy yourself!
If you were a mystery writer, you would totally mess with people's heads, wouldn't you? Agatha Christie did exactly that in The Murder of Roger Ackroyd, and it made her famous. It drove people NUTS. And it's fantastic. Think of the wackiest explanations you can. You never know - it might be right.