Defining your customer is as important as defining your products or naming your business. If you have not yet done a marketing plan within your business plan, then you need to do this exercise. (Here's how to create a complete marketing plan, from your Unique Selling Proposition through advertising and promotions planning.) And if you are starting from square one and need to write a business plan, this Business Plan Outline will lead you through the process step by step.)
The key here is to ask regularly rather than just once or twice. Most of your followers only see a fraction of your posts, and you should be getting new followers every day, so it makes sense to request subscriptions at regular intervals. If you can offer some sort of incentive or even just promise exclusive content within your emails, that helps too.