Express Complete Idea
The subheads are not just meant to divide a writing piece into many so that a reader may find it easier to read, but a subhead should convey and clearly convey a complete idea. It must convey what a reader should expect to read and understand in the paragraph that follows. Use of subheads just to increase length of writing pages or make it look more catchy is not useful at all.
Continuation of Speech/Action
A subhead should not be a standalone line, but should be continuation of the same speech or action that you as a writer are trying to convey to your readers. Consider a between with an idea that you have just presented above. If you are conveying benefits of something, they should continue with a subhead for another benefit or even a greater benefit. A standalone subhead will break coherence of your writing and readers will be lost, and less interested to continue reading the piece.
Plan Subheads in Advance
As you may sometimes start defining your writing piece with a headline first, you can also think of subheads based on the number of ideas you are planning to convey in the piece. Planning in advance about the subheads will also help you approach your writing more systematically.