1. Back up your assertions with pearl-clutching statistics that don't really mean anything: Most YA books are bought by adults, omg the shockhorror. Most groceries are also bought by adults, but that doesn't mean that said adults are ingesting all of that food.
As I said to my father when he was concerned that the dude at Sam Goody thought he was buying that New Kids on the Block cassette for himself, "Um, I doubt it. And also, do you really think he cares? And also also, why exactly do you care so much about what the cashier at Sam Goody thinks?"
2. Base all of your sweeping generalizations on two blockbusters: Good job, you've read The Fault in Our Stars and Eleanor & Park. You are now totally justified in assuming that every other YA book is exactly like one or both of them.
I'm going to start doing that with EVERYTHING now: Rather than read a broad range of adult fiction, covering lots of genres and eras and styles, I shall read two extremely popular modern realistic fiction titles that have been adapted into movies—say, one by Nicholas Sparks and one by Janet Evanovich—and then I'm going to go ahead and judge ALL OF ADULT FICTION on the basis of those two novels. Because logic.
3. Show off your knowledge about the topic by name-dropping titles you've read... like The Westing Game and Tuck Everlasting. So what if neither of those books happens to be a YA title? PIDDLING DETAILS DON'T MATTER WHEN TALKING ABOUT LITERARY JUNK FOOD.
4. When there is pushback about your sweeping generalizations, be pithy and dismissive! Be sure to shake your head sadly and comment about how surprised you are that teachers and librarians and parents and booksellers and authors and agents and editors and publicists AND YES, EVEN REVIEWERS—people who have spent hours and days and years immersed in this rich, diverse, multi-faceted world—are taking this all so personally. After all, when you said "Fellow grown-ups, at the risk of sounding snobbish and joyless and old, we are better than this," you weren't talking to or about THEM.
You were just talking to the idiot masses, who should be reading Dickens.
Read @slate YA piece. It dismisses satisfying endings and sympathetic characters as YA, then says Dickens, king of both, is ok.— Neil Gaiman (@neilhimself) June 5, 2014
5. Rather than taking the opportunity to recommend readalikes, shame readers for what they enjoy! Telling them that they're stupid and immature and incapable of reading critically will definitely make them rely on you for future advice, to trust your opinion in all things, and above all, to PUT DOWN THE YA AND PICK UP THE UPDIKE!