Others have already given some valuable suggestions, so I will focus on what's worked well for us.

(To set the context: We have two businesses around WordPress & WooCommerce, 30+ premium plugins, 250k+ active installs)

1. Free plugin on WP.org, and a Pro upsell. Works really well. But make sure to optimize your plugin's description / readme.txt file.

2. Organic inbound traffic - search engines etc. In depth content will get you visitors. This still works!

3. Building relationships - like Rebecca mentioned. If enough people know you, they may recommend your plugins to others - even if they themselves are not the users.

4. Focus on UX, build a better product. There are just too many plugins out there. You must solve a deep user pain to be able to succeed.

Hope this gets you started..

BTW, I spoke about building and marketing WP plugins at WordCamp Mumbai in 2016.. I think you will still find the talk relevant:
https://wordpress.tv/2017/06/30/nirav-mehta-build-and-they-wont-come-wp-plugin-marketing-tips/

All the best!