Something I learnt years ago was to add picture shelves to a wall instead of just hanging a picture there.
This lets me to have an evolving room design depending on my moods, my latest finds and current fad, or to bring in the changing of the seasons.
I can go minimalist, I can move art or other object around the house and general keep the house interior feeling fresh and it gives me a chance to dust!
For the best results find a shelf that's fairly narrow and has a lip at the front - these are picture shelves - you can find them in many furniture/interior stores and Ikea has a number of inexpensive ones in white or black which are perfect.
From my own trial and errors, for the best impact stick to the same size and colour of shelf and I find it looks amazing if you have white shelves on a white wall, keep your lines straight if your going across the wall as well as up (i.e. don't stagger or off set) so get out your spirit level.
If you want to add a bit of something extra have one accent colour/small size shelf on the bottom row. For example in my last kitchen, I had one wall covered in large white shelves with the lower row made up of narrow black shelves, this made the kitchen look much more expensive than it actually was!
And to gain a cohesive and high impact effect try to keep frames, art work and collectables that have a similar colour or tones - all natural colours and textures or all one colour but with a juxtapose piece -------- and the one great thing with all of this is --------that if you get it wrong your not stuck with it just hanging there! ---move it all around until its right!