Paint the shelves the same color as the wall, or paint the shelves and fire place mantle a different color than the wall so they pop. Painting the shelves and mantle the same trim color will make them look more unified.
Choose items to group on your shelves that have a common theme. For example, shelving can be an excellent place to display a collection of pottery. Even if they're different shapes and sizes, they are unified because they're made of the same substance. Alternatively, use color to unite your items. You can make everything from vases to picture frames look like a group if they're all the same color.
Select fewer, larger pieces to create a bold, stately display. Avoid using a lot of little pieces, which can make the shelves look visually cluttered. The key is to be selective, rather than grabbing everything you like or everything you can find.
Stagger the heights of items. Instead of flanking your tallest matching pieces on each side of a single shelf, place one on a top shelf and one on a bottom shelf. Instead of putting the largest object in the center, put it off center and have things get smaller as they move away from it.
Add more height and interest to smaller objects by grouping them. Alternately, place a small item on top of a small stack of attractive books. Giving them a little stage to stand on visually increases their stature.
Leave space between items. Crowding can make the shelves look cluttered and can take away from everything on them. Approximately one-third of your arrangement should allow for empty space.
Live with your arrangement a few days, and then come back to tweak it by adding, subtracting or swapping things around. Keep making adjustments until you feel it's just right. Don't commit yourself until you love the look.