Most readers skim a long sales letter before deciding whether to actually read it in full, which makes strong subheads essential for keeping their attention moving downward.
Make Each Subhead a Mini-Headline
A subhead should be compelling enough to stand alone, since it may be the only part of that section a skimming reader actually reads.
Create a Trail of Curiosity
Subheads that each raise a new question or promise keep a skimming reader moving forward instead of stopping partway through.
Avoid Generic, Purely Descriptive Labels
"Our Benefits" as a subhead does far less work than a specific, benefit-driven line describing what's actually covered in that section.
Next step: Use the Sales Letter Writer to generate a set of curiosity-driven subheads for the sections of your letter.