#1 remove everything from your shelves. it may be time-consuming but starting witha blank canvas will help you see the space a-new and encourage you to return only what you really love.
#2 assess the book situationa. remove any torn-up or raggedly books (maybe they don't get to be on display...this is where baskets and bins are useful.)b. arrange your books by size. keeping relative sizes together will be pleasing to the eye. the smallest books go on the upper shelves, the largest on the bottom. you don't want things to look top heavy.
#3 place your largest items on the shelf...this may be a grouping of books, storage baskets, vintage gas station signage, all of the above..whatever you want. space these pieces that have more visual impact somewhat randomly in the bookcase.
#4 add a layer of medium sized decorative items...framed prints, plants, a small bust of beethoven, etc. make sure these items have a spacial relation to the largest items...whether they are placed slightly in front or on top. don't leave them floating all alone one the shelf.
#5 finish with any small knick-knacks...seashells, candles, lego men, etc. use these sparingly. you don't want too many little pieces...it'll look busy and messy.
#6 step back and assess. is there too much stuff on one side? too many black things on that one shelf? is it too symmetrical? don't worry if it doesn't look perfect right away...this is a process and now is the time to mess around with it.
Enjoy and happy decorating.