So you've chosen some art, framed it, chosen a location, now how to hang it to make it look its best?
A large, bold piece should be placed with enough space around it that it isn't crowded by other pieces or the furniture around it. However, a single small piece hung in the middle of the wall on its own can get lost. A pairing, trio, or grouping of themed pieces can be a much more interesting arrangement. By themed, I mean pieces with something in common, either by the same artist, or the same subject matter, or a similar style, or common colours. They should compliment one another, not be 'the same'.
If grouping pieces, try not lining them up in a simple row. "Salon" style hanging, putting pieces of different sizes close together at varying heights, not just beside but also above and underneath the pieces near them, gives them importance and visual interest. You can fill some or all of a wall this way.
If hanging pieces separately within the room, remember they don't need to be the same height. This will depend on the size of each piece, and what's near it in the room - is it over the sofa, between chairs, next to or across from the window, etc. Get someone to hold it up to the wall and stand back, trying a couple of different heights and spots in the room, as well as different amounts of space between any grouped pieces, before making holes in the wall.