Here are the points which would be helpful to increase the RSS subscribers of your blog:
1) Put a Sign up button in every post and page of your blog: Putting a sign up button typically in the sidebar is a great way to increase your subscribers. Say, if anyone likes your post, they will look for a sign up button to subscribe to your posts. So, why not to include in every posts. The idea is to make your sign up button visible to your readers.
2) Use a subscribe remind plugin: This is a nice plugin as it reminds your visitors to sign up for the RSS feed. I guess, putting this in the bottom of your posts is the best place as the reader has just finished reading the article and the next thing he would like is to subscribe (Of course, provided he or she liked the article).
3) Stick to a strict niche: You have to make sure that your blog is on a tight niche. Now, say you like SEO and subscribed to a SEO blog to receive contents related to SEO. Now, if the author starts posting about Basketball for a couple of weeks, then definitely you will unsubscribe from his feed. That’s the reason a blog which is on tight niche has lots of subscribers.
4) Submit your website’s feed to a list of RSS feed submission sites: There are many sites where you can submit the RSS feed of your blog. It’s a no rocket science to understand this as you are just increasing the visibility of your feed.
5) Share your RSS feed on social networks: You can share your feed address on Twitter, Facebook, Google+ and other social networks which you use. This is also a good way to increase the subscribers.
6) Put the RSS feed in your email signature: We send many emails to people on a daily basis. You can create a subscribe button in your email signature. This is also a good way to increase the subscribers.
7) Use Feedburner instead of your default WordPress feed: There are plethora of options in Feedburner and you can customize anything. When it comes to feeds Feedburner is the number one choice.
8) Show your feed count: In Feedburner you can select the show feed count for your feed. When people see that you have many subscribers then they think that your blog is credible. It’s like: money makes more money kind of thing and it surely works.
9) Add a footer or header button: Many people look for footer or header buttons for a RSS feed. You can add it as per your wish.
10) Allow an email option: Many readers don’t use Feed readers. You can give them an option to subscribe via email. You can activate this in Feedburner and add a widget in the sidebar. Statistically, more than 50% of my blog subscribers in any blog are email subscribers.