There's a popular view of writers that says they scribble down inspired prose whenever the muse hits them, working frantically whenever inspiration arrives. This might be the way to capture new topics and ideas at the beginning of the writing process, but finishing your books means making a commitment to writing on a regular basis, whether you feel like it or not. If you're used to only writing as a hobby, when you're in the mood, writing on demand might feel like a foreign process. Use some tricks and techniques to turn your mind in a creative direction more often, and you'll increase your word count considerably.
When the muse hits, your words come smoothly and the story just evolves right in front of your eyes. It's a great feeling when it happens, and you can force your mind into that mode more often by giving it little creative problems to solve. Look for daily writing prompts online to warm up your creative muscles. Doing a two-paragraph quickie exercise every day can teach your mind to move into that creative mode that produces great fiction. Much like warming up the muscles makes working out easier, doing a warmup writing prompt can ease your transition into a smooth writing period.
Sign up for NaNoWriMo (National Novel Writing Month) or one of the monthly blog challenges held online. Making a commitment to write every single day can galvanize your writing habits. If practice makes perfect, then writing every single day should increase the amount of good writing you're doing every month. Writing to a designated word count each day also forces you to acknowledge that, while you want your final product to be spectacular, not everything you write will be golden prose. And that's all right.
Know one thing: the perfect is the enemy of the good. If you're waiting for your muse to strike before putting your fingers on the keys, it's likely because you think you shouldn't write anything at all unless it's going to be great. The problem with this concept is that it's completely wrong and is the opposite of what every published writer does. Your favorite author writes junk every single day. First drafts are inherently bad. The reason editors exist is because writers need the creative freedom to try out words and phrases, experimenting to see if they work or not.
The reality of book writing and publishing is far from the romantic notion of the inspired writer, but it's how you get the words on the screen and the books online for sale. They say it takes at least 21 days in a row to create a habit. See if you can make your own habit that turns into a profitable venture.
Keep the Faith and may the Force be with You!