How do you design a show banner that doesn't look dated in a year? Three thoughts:
One: Change your banner image every time you start a new major project. Even if the look and feel of a new book is not substantially different from previous work, even if the main character hasn't changed, you can reinterpret that look, take a few risks. Pick something really striking and poetic, an image that sums up the whole project.
Two: Change your banner image every time you completely redo your website. Repeating the images you use to draw people to your site helps get it into their heads like bright, sparkly brain-chiggers.
Three: Change your banner every time someone puts a foot through it during tear-down at a show. This is certainly the most economical option.
No, really; finding a balance between sticking to a really striking, iconic image, especially one that people are used to looking for to locate you at shows, and making your look fresh and new to reflect evolution of your style in art and story, it's tricky. Movie posters oscillate between abstract-stylish-icon, like the poster for VERTIGO or JAWS, and Big Faces Of Stars, which show people the pretty pretty mugs of the angular love-beasts they follow into the theaters. Posters and covers have their tropes and iconography. Learn 'em, use 'em, break 'em if they don't serve your needs, tastes, and philosophies. When I made my banner image, I used a character that is often but not always my protagonist. I made an essentially life-size image of him; people like to take pictures with him. He's in his natural habitat (dirt) and there are other elements you can't see unless you walk up to the table. I hope it suggests that there's more to see than first meets the eye.
BUT I've trotted it around for years, and I'll be thinking of a new one starting....