Personally, I find that motivation to keep reading isn't that hard because writing is so deeply ingrained in me that I can't NOT write. But when it comes to staying motivated to finish one specific story, I think two things that helped me finish my novel "Damsel in the Red Dress" (and keep working on it's ongoing sequel) were the deadline and actually releasing it. Because I started the novel for a challenge, I felt it would be a waste of time to quit before I finished. Also because I'm releasing it and have people reading and supporting it, I feel accountability to make sure they actually get the chapters they're waiting for.
When it comes to keeping motivation in general, I think working on several different projects can also be helpful so you can work on different ideas when one starts to burn you out, but not just drop writing altogether for long periods of time. Reading and watching a lot of stories, and going out and seeing new things is also helpful, because the more interesting things you see, the more you'll be inspired. These are just things I've found inspire me personally, and it's always easier to write when you're inspired.