Sub-headings need to be seen as key parts of your text.
Sub-headings create focus
Sub-headings act as the landing lights for your reader coming in. Without them, all the reader and the search engines might see is a mass of dark text with no guide as to what are the most important parts for them to read.
Sub-headings make it easy for your reader
What we have to do is ease our reader through the text in the way we want them to go. Make them work hard and they're likely to bail out.
How do you produce effective sub-headings?
Sub-headings provide exactly the same service as do headlines. They are simply further down the page. Headlines are there to coax you to read the first line of the text. Sub-headings are there to coax you to read the next section.
Think of the story you want them to read
Guiding them through the story via the sub-headings means that if they dip in half way down the piece, the sub-heading where they dip in should be as compelling a part of the story as is the headline.
Pull them into the conversation via the sub-headings
Using open questions in the sub-headings starts them thinking about the conversation that's happening in your piece. Teasing them and intriguing them with interesting facts is another way of doing it.
You only have one chance to attract their interest. Hiding the most important or fascinating facts deep in your text won't help you gain their interest.
Be bold, be up-front and be genuine with your sub-headings. Construct them properly and they'll work hard for you.