I've been writing all my life, have 3 published novels, 2 collections of short stories, and 3 non-fiction books (under another pseudonym). And I still strive to improve my writing with every stroke of the pen and each tap of the keyboard. There are all kinds of things you can try in an attempt to improve your writing, but I have a few favorites. I do most of these things daily and I firmly believe that they make me a better writer. Try them out for yourself and see if any of them can help you improve the form and function of your written work.
Create Writing Exercises
Writing exercises can be just the thing to help you improve those things that need to be improved. Perhaps you're fictionalizing non-fictional events while writing your novel. You can practice this with writing exercises designed to create fiction from non-fiction. If you're like me, you don't write a lot from the first person point of view. I'm on the verge of tackling my first novel written in the first person, so I've been practicing by reading works by other authors, picking a character, and writing a chapter or so from the viewpoint of the character I've chosen. Pick another book and repeat.
Maybe your problem is the excessive use of passive voice ("to be" and its derivatives generally tell you that you've slipped into passive voice). If this is your problem, practice writing passages with no passive voice at all.
Many young authors rely on adverbs and adjectives to tell their story. If you're overusing these words (watch for words ending in -ly; they're the biggest culprits), you should practicing writing without them. Write a passage that appeals to you, one that is at least 500 words long. Then go through that passage and eliminate all the adjectives and adverbs. Now strengthen the remaining language so that you don't need the words you took out. I'm not saying that adverbs and adjectives are bad. I use them all the time. But I use them with care, inserting them where they'll have the greatest effect. Using them too often weakens your writing, so practice until you no longer rely on them. Then they become a tool rather than a crutch.
Practice your weaknesses. If you're not very good at dialogue, if all your dialogue sounds the same, practice distinctive dialogue. If you've never written a novel and find yourself a little intimidated, practice writing short stories first. I start every day with a writing exercise to get the juices flowing. Try it and see how it works for you.
Brainstorming to Improve Your Fiction Writing
If you're writing a novel, you have to have a seemingly-endless stream of ideas. This can be difficult as you progress and the word count grows. But when you're having trouble with a scene or don't know where to go with your plot, try brainstorming, sometimes called word association.
Sit down with a pen and paper and write down a word or phrase that represents the scene or plot you're working on. Then let other words and phrases come to you, writing them down without regard to how they might apply to your story. You never know what will come of it.
As an example, I had a young woman chained in a cave. This young woman has magick, but she's so injured that she has not the strength to use it. She's good with a sword, but she's chained and doesn't have a sword at the moment. Where's the guy with the lock picks when you need him? Oh, that's right. He's 1000 miles away. And anyone else who might have been able to help her was either captured themselves, too far away, or unaware of her plight. How, exactly, am I going to get her out of this one? I could go back and rewrite parts of the story, but I'm happy with what I've written.
Time to brainstorm. Since the problem is those damned chains, that's the word I started with. Chains. Trapped. Cold. Metal. Sharp. Blood. Blood magick.
There it was. Just a few words in and I'd come across what I needed. What if the chains had cut into her ankles? She's now bleeding. She's a creature of magick. Magick flows in her blood. What's a little blood magick between friends?
It took some ironing out, but I managed to get my heroine free without having to alter the story too much. She was rather the worse for wear, but she was at least in one piece. Mostly.
You can use this same technique when you run into problems. This will help you improve your story telling and come up with new ideas. Or perhaps draw out that idea that was hiding in the back of your mind just waiting to be set free.
Timed Writing as an Exercise
Have you got a timer of some kind? A stopwatch? A microwave (these all have timers on them these day)? An egg timer? Then you can do this little exercise. If you're being plagued by writer's block or find yourself procrastinating, you should grab a pen and paper and sit next to your timer. Give yourself about 10 minutes and just write. Write anything, but write constantly.
Don't worry about grammar, spelling, or punctuation. In fact, don't worry about anything that's appearing on the paper before you. Just let things flow. If you can't think of anything to write, start with a single word. Repeat that word on the page until other words come to your mind. Eventually, you'll be writing something intelligible, though not necessarily something good. But, over time, you'll learn to sit down and write, and the stuff you write will get better.
Is it important to be able to write on command? It is for me. I'm a novelist by profession. This means that I made all my income from writing books. Royalties are my bread and butter. And I have two small children to support all by myself. So if I can't write, I'm taking food out of my childrens' mouths. Not literally, but every day I waste is another day that I could be working on a project that will put food on the table when the royalties come in.
If you're hoping to be able to actually make some money writing, you have to write on command, and you have to do it well. The only way to do this is to practice.
Believe in Your Story
If you really want to write well and continue to improve, then you need to have some faith. You have to believe that you are writing a story to the best of your ability. You have to convince yourself that you are perfectly capable of writing a story that others will want to read if you just try a little harder.
You also have to banish those little thoughts that bog writers down. "Why can't I write anything decent?" "This is garbage!" "No one will want to read this!" "I am a terrible writer." All of these thoughts are detrimental to your work as a writer.
Every time you catch yourself thinking these things, change the thought to something positive. "My writing is getting better every day." "People will love this!" "I am a writer." These thoughts will slowly change the way you think and improve you ability to write and tell a story. It works, but only if you truly believe in what you're telling yourself.
You do create your own reality, especially as a novelist, so practice and believe in yourself and you'll find your own writing improving daily.