1. Keep a consistent posting schedule (once per week, every 3 days, etc); a lot of people will enjoy a story more if they can look forward to when the next chapter comes out. This applies to overposting as well; if it feels like a job to keep up with the story, people will read it less.
2. Engage with your readers when they comment. The general idea is that you get more of the behavior you reward, so by showing that you read their comment, you are more likely to get more comments. It can be as simple as a "thanks for reading!" The idea is just to show that you appreciate it. Something I have noticed is that people are less likely to comment if nobody else has commented on that chapter, though that's just an observation.
3. This is more of a style thing, but make sure to break your writing into different lines; people seem to dislike stories if it is written in whole paragraphs. Not necessarily every sentence is a new line, but for dialogue and when moving on to a new topic. Here is an example from a pretty popular story: