First Second produces some of the most beautifully designed graphic novels on the market today, and in this interview at Artist Abbreviated, designer Colleen AF Venable explains the cover design process from start to finish. It's definitely a combination of inspiration and perspiration:
If you count of the number of thumbnails we did for those two books [Brain Camp and Kristyna's Ghost] it is over 40. Thumbnails are the most important stage. Without an awesome thumbnail, one that works even if seen at a single inch tall, your cover will never really shine. We then go to pencils and I obsess in that stage a bit more, eyes need to be exactly right (the thing that draws viewers in the quickest), proportions need to be perfect, space for logo should feel deliberate, not an afterthought. It can take months before we go from perfect thumbnail, to perfect pencil.
She also talks about her love of spot gloss and other special effects, her dream of someday designing a wordless cover, and the day she was discovered as a designer. There's a nice gallery of images to go along with her discussion of the types of art she finds most compelling, so this is well worth a leisurely read.