Here are a few ideas for styling your shelves...
A Certain Theme
The bookshelf above uses a theme - nautical decor.
Perhaps sea shells can be gathered from vacations, and are mementos you may wish to display so you can revisit the memories.
A Time & Place
You might choose vintage collectibles to showcase on your shelves, or an eclectic combination of things.
Some decorators like to turn the books around so that the spine is not seen. The above shelves uses neutral colors mixed with green.
The most important thing is not to make your shelves look too cluttered. Showcase the things that are most important to you.
Rotate your collections by holiday or season.
Bookshelves serve as both storage and decor. Add and edit until your shelves look balanced and pleasing to the eye.