I tried five different business models over the last two years to earn money with my writing habit, but there are only three I would advise to indie writers: book writing, freelancing, and business marketing.
The other two are affiliate marketing and donations, but the former requires too much traffic to be sustainable for most indie writers, and one could argue the latter isn't a business model.
Book writing made me about $100. I didn't do much to market the book because it's still a huge disappointment, but I'll eventually get back into it.
Becoming an indie author feels almost natural if you're already writing on a daily basis, and it can lead to the creation of many income sources: audio books, internationalization, movie rights, speaking gigs... the more books you publish, the bigger the opportunities.
Freelancing as a technical writer has been more lucrative though. I made about $2500 in half a week's worth of work, averaging a $90 hourly rate. If you have a specific niche and learn to leverage the power of the Internet, sky is the limit. Companies from all backgrounds need marketers, and content marketing is still considered a must-have. Competition can be fierce, so you'll have to be able to prove your skills as a subject-matter experts and your writing proficiency. Most of the time, writing in a clear and concise English is 75% of the work (trust me, I've been working with UK and US startups, and I'm a French guy, so anything is possible as long as you can prove you can write).
For me, business marketing has been the most productive approach. It's about using your content to market your product and/or service. Even though it's not a direct transaction where you are paid to write, it's still effective. I'd say I made $2700 from my daily writing habit, simply talking about my products and the way I work on my tech business.
In each situation, it's all about getting started: you just need a platform, regularly publish new content, and do your best to share it.