Forgettable (adj.): Easily forgotten, esp. through being uninteresting or mediocre.

Step 1: Set the scene
Step 2: Drop some great characters there
Step 3: Make something fantastic happen

Ick. No one wants that for their writing. One of the best ways to avoid this is through the help of your early (beta) readers. You can’t just go asking if your book was boring; few of us are lucky enough to have the kind of brutal frankness among our most dedicated readers. Instead, phrase your question in reverse: “What stood out to you?” “What was your favorite part?” “What scenes really wowed you?” If you can’t get much out of your readers on these topics, or you can hear the rustling of the straws they’re grasping at, take the hint: you haven’t captured their imagination.

Even if you do get some positive feedback when you ask, there are still ways to burn your story into your readers’ brains.

Settings

This is the most visual way to leave an impression on someone. To be memorable, you have to make your reader work. You can paint them the most vivid scene with wonderfully crafted prose, but if the reader can flip through their mental image gallery and pull out the equivalent of a stock photo to use as the image they associate with it, you missed your mark. They may remember your wonderful writing, but the setting … maybe not so much.

You need to focus on jarring their imagination loose from its shackles. They need an image that they have to create from scratch with only your words to go by. The city of New York has been done to death in every medium; you can probably picture it under alien attack with nary an effort. Give the reader an image of those skyscrapers shaking and pulling loose from their foundations and rising into the sky, leaving a sprawling plain filled with concrete pits and they will have to work to see it.

A lot of your settings will be dictated by the needs of the story you’re telling. When possible though, try to veer from the well-trod paths. Pick something that is mentally jarring to force the reader away from things they know.

Characters

I’ve read books and when asked about them later, could not even recall details of the main character. If you can spend 200+ pages (and sometimes many more) without developing a lasting memory of the protagonist, don’t blame yourself. They author has failed to give you a reason to remember them. How can you avoid the same thing?

Appearance: Strange as it may seem, this is probably the least important aspect of a character as far as remembering them. While you don’t want to stamp every hero from the same square-jawed, muscle-bound mold or describe every heroine as a runway model beauty, appearance ought to be secondary. Unless you devote a lot of text to describing them throughout the book (at which point readers may remember them for the wrong reason: being annoyed by them), appearance is something in the background.
Mannerisms: This is a better one. If there is something that your character does that really sets him or her apart. Quirky and a little weird is a good combination. Things that try to make the character seem “cool” are as likely to backfire as help. Especially annoying is usually effective, but be prepared for it to be a negative kind of attention (better for villains and minor characters than a protagonist).
Background: Farm boy. Runaway. Prostitute. Secret agent. Orphan. Ex-cop. I’ve probably just crossed out the protagonists of half of all fiction. There are so many possible stories behind a character that making one stand out takes a lot of work. First, the character background probably isn’t tied into the main plot. Second, you need something that both works for the story and is plausible at the same time. Just make sure you’re not breaking the believability of the character.

Plot

This is a whole different game. There is no trick to an unforgettable plot. The scenes that contain the best twists, the biggest surprises, the best payoffs for coming to the end of a long book, those are all things that you should be striving for. The best you can do is to think about them and what makes your story unique. Focus on setting it apart even further, accentuating what you are doing that other books aren’t.

Advice: Study the Movies

Hollywood has its faults, but there’s something to be said for their ability to produce a spectacle. Think of the scenes that capture your attention, the characters who have become legends, the plot twists you still hear people talking about years later. The sinking of the Titanic, Indiana Jones, Rosebud.

Why look to movies instead of books? Well, for one it’s easier for disparate groups to discuss movies, since even without seeing them most people are familiar with the classics and the blockbusters. Also, movies have a very visceral ability to suck people in and leave an impression. Being able to write the sort of story that people would want to see told on screen is a good way to keep a focus on engaging the reader and having it be your work that leaves that impression.