Duplicate Post.

White and blue decor for a simple clean look.

Nothing that overhangs the edge of the shelf, I just noticed the things you have that make your shelf look way too narrow.

https://www.stonegableblog.com/decorating-a-bookshelf/

Fill it with books, then nothing else can invade.

If you need to actually store things there that aren't decorative you could have doors made for it.

The shelves appear to be quite shallow, which makes them more suitable for small collectibles rather than storage. Think about using natural finds from your area such as rocks, shells, pinecones, driftwood, etc. Here’s a rough mock up from a photo I found online. The background color similar to your wall color makes me think it might be nice to paint the back of the cabinet to match the walls.

Do you have another logical place for the things that are on the shelves?

I would either fill it with books, or get matching bins or baskets that just fir the shelves.

Fill the shelves with boxes or bins, so the shelves can still be used but the boxes/bins will give it a more organized look.

"they end up with mess and clutter"

My husband and I always joke about passive voice home responsibilities. "The milk was not put away" instead of "I left the milk out". I think this is a similar issue. The shelves end up with mess and clutter because you mess and clutter them up. Think before you put something on the shelf and put it somewhere else.

Maybe these shelves being cluttered are a symptom of you not having enough well organized storage elsewhere. Is there another home for everything else that is currently on these shelves?

The 4 shelves in your bookcase is adding to the busyness.. Are your shelves repositionable? This example is a little busy to me, especially all the picture frames on the top, but will maybe give you some ideas (books, greenery and natural items like wicker and wood).

Pretty baskets and boxes with lids should help.