Nicky Haslam Design
To many people it may be unthinkable to know that more people than they think forget about dressing the walls.  They either get scared or intimidated by the thought of Gallery or salon walls or they just don't know to fill the space.  I guess a simple rule of thumb is to get rid of the idea of perfection end result and any rule for that matter, and consider the conditions of the walls first of all.  If it's in good condition maybe a good coat of paint or an elegant and simple paint technique before hanging one statement piece would be enough, as in Nicky Haslam Design in the top image, the walls are so beautifully done that a cluster of framed pictures would ruin the whole look.  Beautiful objects such as big mirror, one big painting or an exotic textiles are also simple options.  Of course a gallery wall is always a one step up if you decide to go for it.  Enjoy!