Gallery Walls are a great way to fill a wall, and it doesn't need to just be art. You can place clocks, mirrors, or any interesting object. The key is to keep the spacing uniform.
So maybe you don't have a large wall to fill. Maybe the problem child is the awkward medium size wall, in which I would suggest stacking some art vertically like the pics below.
Next would be selecting a variety of accessories. You want them to be related in some way, like the image below, most of the accessories are of the same color tone. But they also need to different to have interest, and this is where you can play with things like scale and texture. In the photo below, I feel that they are able to get away with so many objects because the artwork behind it is so simple. Your eye has a rest with the neutral canvas that the accessories don't feel overwhelming.
Lastly, I think words or letters here and there can add interest, subway art is a great example of this. With words I think you need to seriously restraint yourself from going nuts. But I do like them in the right setting. I will have another accessorizing post because this is a seemingly endless topic!
Photo Credits: apartment therapy, http://www.11magnolialane.com/2014/06/02/diy-photo-gallery-wall/, http://www.domainehome.com/simple-home-design/, http://www.preciouslyme.com/home-decor/angie-hranowskys-home-4688, http://www.bloodandchampagne.com/page/8/, http://www.veranda.com/designers-ideas/ralph-lauren-home-accessories, http://brabournefarm.blogspot.com/2012/02/little-tables.html, http://www.nellhillsblog.com/?p=2163, http://www.thehouseofsmiths.com/2011/05/large-address-subway-art-for-gallery.html, popsugar.com