So I was discussing query letters with a friend the other day, and there's one particular portion that a lot of people get wrong and yet almost everyone includes it in their query letter.
Comparisons.
This isn't a debate about whether or not you should even put one in your letter. I think if you do, you need to do it well or else you're shooting yourself in the foot. Some people think it's a bad idea to have a book comparison in your query letter ("It's like Anne of Green Gables meets Predator!"). Some people think it's very beneficial to have one in your query. You have to decide what is best for you - I'm not here to tell you that. Rather, I am here to point out some bad comparisons.
The Stone Age Comparisons - Where you compare your new manuscript to things that came out a jillion years ago. "It's like GONE WITH THE WIND meets THE FAR PAVILIONS!"
Why this is bad - This tells an agent that you haven't read a book in your genre in approximately 80 years. Seriously. When picking comparisons out, your best bet is to pick books that were released in the past 10 years. This shows you are current on your genre, you know what's selling to editors, and you know what kinds of an audience you are pulling in.
The Harry Potter Meets Davinci Code Comparison -- "It will appeal to people that like THE HUNGER GAMES, TWILIGHT and the Harry Potter books."
Why this is bad - Listing the mega-hits of any genre doesn't show that you know anything about the genre itself. It just shows that you know an extremely, extremely popular book. Pick something that wasn't necessarily a mega-huge-bestseller. Pick a smaller book that won awards or has the same kind of tone. EVERYONE is going to compare their book to Twilight, and you want to stand out. Because who DIDN'T like The Hunger Games, Twilight, or Harry Potter? Seriously.
At the same time, don't be so ridiculously obscure that an agent won't know what book you're talking about.
The CrossGenre that's REALLY CrossGenre -- You think your book perfectly encapsulates that narrow blending of HP Lovecraft and THE DIARY OF A WIMPY KID.
What this tells an agent - Who is going to buy this thing, really? Too cross-genre means that your query is going to come across as weird, or too narrow of a market for an agent to be interested in anyhow. Pick things that are relatable, or target the same audience. The average person reading THE GIRL WITH THE DRAGON TATTOO is not the same type of person who might be reading PRINCESS ACADEMY. (They might be, but odds are against it.)
The Two TV Shows That Were Cancelled Way Way Early - It's like Dead Like Me meets Dollhouse!
What this tells an agent - Yes...because those two were so very, very successful, right? Riiiight. Picking TV shows that were cancelled after one season means there was NO AUDIENCE FOR IT. Don't compare your book to things that failed. Which brings me to...
The Comparison to TV Shows/Movies/Video Games - My book is just like Battlestar Galactica if it was set in the Final Fantasy Universe!
What this tells an agent - You...do realize you're trying to sell a book, right? While I realize that it's so so so hard to not compare your book to that one perfect movie that encapsulates the vibe you're wanting (been there!), resist if you can. You'll impress an agent more by telling him/her that your secondworld fantasy is like Tamora Pierce more than it's like Hercules: The Legendary Journeys.
The Two Things That Sound Nothing Like Your Book - You've just talked for three paragraphs lovingly detailing how your book is a medical romance involving the lives of two rival doctors who are hot for each other...and then compare your book to Dean Koontz and Diana Gabaldon.
What this tells an agent - Either you have no clue what your book is about, or your query is completely misleading. Both are bad.
What you should do when comparing your book? Pick two things in your genre (or that might appeal to the same audience). Preferably books that had a moderate amount of success and have a similar tone. Explain why you are picking these things - do they share a similar brand of humor? Are they both about vampire princesses? Splain, Lucy. Splain. But keep it short. Show that you know your genre and the current market, and how your book would fit perfectly there.
Your mileage may vary on all of this. These are simply my opinions.