The number of RSS subscribers is often used to determine which blog is more popular. Here are some tips to make your number of RSS subscribers grow:
-Organize a contest in which one of the requirements to enter will be to subscribe to your RSS feed. According to many bloggers it really works. I know bloggers who subscribed to at least a few RSS feeds, because they wanted to participate in contests. However, you have to make sure that the contest will be attractive enough for your readers.
-Reward your readers for signing up. You can offer them something that will not increase your costs like, for example, an eBook. Such an eBook should be relevant to your niche.
-Offer to your readers the possibility of subscribing to your RSS feed through email. Some visitors prefer email subscriptions, and no matter how hard you might try, they might still not be interested in subscribing to your RSS feed any other way. If you allow this way of subscribing, you are going to be able to tell who your subscribers are as you will know their email addresses. If you want, you can even personally thank them for signing up.