Styling shelves is like creating a little art installation. An overall composition, palette, and narrative must be considered. Styling a shelf has become a way for individuals to curate their life experience and collections. There are three rules I try to go by when putting together a bookcase, the first is to consider the colors of my books and objects and how they interact. The second rule is to vary the scale of my objects greatly, avoiding too many small objects. The third rule is to vary the texture of the items on display.
Color, generally if you have brown bookcases you need to search out books with lighter bindings and color. People often do not have a variety of colors in their books so in this case a great solution is to paint the back wall of your bookcase with a pop of color. The next aspect of color to consider is to try to bring colors from all areas of the color wheel in, and you can do this without ruining a calm or focused palette by being subtle with those shifts. You do not have to have a bright yellow object on a shelf next to a red, but you could put something in that is brass or gold to stand in for the yellow end of the spectrum. You also want to make sure that if you are placing a decorative object on top of a book or in front of a book that they contrast in hue or value.
Scale is very important. When you look at a bookshelf with objects and books all of a similar size your brain does not stop to ponder anything, it just says "rows of books" or "shelves full of a bunch of stuff". A big bright vase and a small sculpture, a big decorative mask and a tiny picture frame, mix it up in scale and each object helps to enhance the importance of the other objects instead of canceling them out.
Texture is something most people do not consider, but we are all tactile people and whether we consciously think about it or not we do respond to texture. So make sure to incorporate soft, sharp, shiny, rough and any other textures you can find around your house.
The most important thing is to have fun! Go through old shoeboxes, don't be afraid to display a childhood toy, you will be surprised to find you have lived a rich life and already have so many things to display that you had never thought of using before! Find some old sheet music, frame it and put it on a shelf. Every time you look at it you will be reminded of childhood piano lessons and the beauty of classical music. Wonky pottery made in school can be a great reminder of still finding fun in failure. Just play and you will be very surprised at what you can do!