Genre conventions are the whole game
Every genre has a visual contract with its readers. Epic fantasy: lone figure, vast landscape, metallic serif title. Thriller: high-contrast photographic imagery, bold condensed sans-serif, often a fractured or blurred motif. Romance: warm palette, figures or symbolic objects, elegant script accents. Literary fiction: abstract or typographic-led, generous white space. Prompt the convention set of your genre explicitly — a beautiful cover in the wrong genre language kills sales.
Study the top 20 covers in your category, list the shared elements, and put those exact terms in your prompt.
Design for the thumbnail first
Online, your cover is discovered at about 100 pixels tall. Title must be readable and the central image must parse at that size — prompt 'bold high-contrast title treatment, single strong focal element'. Generate, zoom out to thumbnail scale, and only shortlist covers that pass. Print considerations (spine, back cover) come after the thumbnail wins.
Series branding and full wraps
For a series, lock the layout skeleton — title position, typographic style, palette logic — and vary the central image per book. Readers should recognize book three from across the room. For print-ready full wraps, generate the front cover art with extended background ('scene continues to the left for the spine and back'), then assemble the wrap in your layout tool at your printer's exact trim size.