There are so many ways to get rid of repetition, and I highly recommend going through your manuscript at least once with a focus on this oh-so-annoying writer habit. I mean, we all do it. It's unavoidable. That's what revision is for.
Let's start small and go big, ya?
Just like words lose their impact in repetition, punctuation does as well. My personal crime is the overuse of dashes, but it's important to watch out for ellipses, semi-colons, exclamation marks, etc. I get so crazy with dashes that one of my friends said I was allowed just one every other page at the most.
As you go through your work, look closely at the punctuation you favor and consider varying it up.
I think word repetition is the one we're most aware of. I personally can't stand to have a repeated word in a sentence. I avoid it at all costs. If it's a "big word" or a made up word or a "bad" word or something punchy/funny, it should be used sparingly. They have impact—they also stand out much more when repeated.
A reader will remember you describing something as "lenticular" seven times, while "round" tends to blend in more, you know?
And then there are personal word ticks. Mine are "just" and "even." I usually have to remove over 100 of each from my rough drafts. I try to watch for it while writing, but I still don't notice how often I do it.
Sometimes with get into structural ruts. Look at your sentences—are they all the same relative length? Do you use certain phrases more than you should? Are your sentences' grammatical structure all the same?
I tend to have an extreme love of participle phrases. Sometimes my early writing will look like this: "He walked to the door, wondering who it could be. He opened it, eyeing the strange figure. The man wore a top hat, dripping wet from the rain."
Same. Structure. Over and over. That is not good. Sentences need to be as varied as words, if not more. Monotonous grammar kills.
The layout of your words is far more important than we sometimes realize. Just like a bunch of same-length sentences is boring, so it a page full of same-length paragraphs. Our minds seem to thrive on variety. Visual, mental, physical, what-have-you.
I tend to write in three-line paragraphs. I have no idea why, but I do. My pages will often be three-line paragraphs all the way down if I'm not paying attention. Other writers tend to abuse the one-liner, having one every other paragraph. Some writers write EPIC paragraphs that make the entire page one miserable block of text.
There's no right or wrong, as long as you switch it up.
Yes, even chapters can benefit by some differences in length. Look at your chapters. Are they all ten pages long? Is there a way to make some snappier and some longer? Perhaps. Keeping the reader guessing is good. A short chapter may propel them to the next. A long chapter may keep them reading and invested.
Not saying you should have one-pagers when all your others are twenty, but they certainly shouldn't all be the same.
Watch out for how you begin and end chapters as well. I tend to have a lot of chapters ending with someone leaving, which is not only repetitious but gives a reader the chance to put down the book. Look for different ways to begin and end chapters that will keep readers intrigued.
Another place to watch for repetition is in and around dialogue. Are you explaining what your character just said? Is this conversation similar to any others? Is there new information given, or is old info regurgitated in a different way?
Also watch for character actions. Does your MC solve problems the same way every time? Is he or she facing the same problem over and over with no real change or complications? Is your character growing and making different choices, or just staying the same?
Be aware of setting repetition, too. How many scenes take place in the living room? At school? In the car? Perhaps some of those can be changed to more exciting/new venues.
As you've probably gathered, the key here is variation. It's essential to good writing. If you are over-looking repetition, I highly recommend you mend your ways. Taking a closer look at all these things has greatly improved my own writing, and I hope it helps you, too.