Rule #1: In late, out early

"In late, out early." This is one of the most well-used and well-known writing sayings today simply because it should always be followed. (This is especially true of YA fiction and younger genres, though apparently not so much for adult fiction if most published works are any indication - but don't get me started on that.)

This saying literally means you should begin each chapter (or scene) either at the last possible moment before or right after something interesting happens - and then end the chapter/scene right after it's over. No backstory infodump, no unnecessary description, no filler. Just the best bits of the story - not much more, not much less.

Backstory is mainly for the author's information only. Plot and write whatever backstory you need to, but unless it's absolutely necessary to the story, you shouldn't use it in the book. If you do need to explain something about a character's past, or perhaps a bit of world history, mix it into another necessary scene - don't just infodump it wherever it "feels right." The same goes for description. Weave it into the narrative using the character's senses. Don't "set the scene" (start a scene with a block of description) unless the scene truly needs setting (and in most cases, it doesn't).

The only time filler is appropriate is for pace padding (so the story flows properly), and even then filler should only be necessary information to further the story. I just used a bit of filler in ID - I felt the need to expound a few paragraphs on EP and AD's history together while they were driving to NYC. When I got done, though, I felt a bit dirty for throwing in the part-flashback/part-infodump scrap of filler, and will most likely trim it down to bare facts in editing.

Every scene, paragraph, sentence, even word, should inform the reader of necessary information and move the story along. If it doesn't do this, it doesn't belong. I don't care if it's your favorite scene in which Martin goes to the Laundromat to wash his socks and runs into a batty old woman. If this old woman (or his socks) doesn't end up predicting a dastardly event in Martin's future, and instead simply annoy him, then the reader doesn't need to know about it. I don't care how many hours you put into crafting the old woman (or his socks). (But if it's already written, don't toss it! Literary Deleted Scenes now have a place: on your website.)

Now, I'm not saying that you should fret every time you begin a chapter/scene, worrying if you're starting at the right point. The First Draft is for getting down whatever flows out of you. If Martin suddenly goes to the Laundromat, let him. He may end up just washing his socks, or the batty old woman may predict his death. It isn't until later drafts that you need to worry if something furthers the story or not. Write whatever comes to you - and then prune it like a bonsai later.