Above: K West Images (living room).
What follows are merely suggestions. Do what's pleasing to your eye, and it will all work out. It's not an exact science.
The first step is to figure out exactly what you'd like to display. In a child's room, you might include colour copies of the covers of their favourite books, their own colourful art or inexpensive paintings or prints from Etsy. In a grown-up space, a mix of black-and-white photos and colourful original art is a tried-and-true combination (see Francis' living room wall above). Just be sure to select pieces that are meaningful to you and your family. But do limit the palette somewhat; too many varied colours can start to look busy.
Choose frames in a variety of sizes and in two or three different finishes for the most visually appealing effect. In Francis' living room, she used unframed canvases, black frames and one gold frame, which works thanks to the complementary shades of gold in some of the other featured art and one of the throw pillows.
There is no right or wrong way to hang art in a gallery wall. In fact, some quirkiness in the spacing and arrangement can actually add to the overall effect. Search blogs, decor magazines and Pinterest for inspiration. Then lay everything out on the floor and start moving things around, making sure like objects aren't too close together. Snap a pic of your floor mosaic so you'll have a visual reference once you start putting nail to wall.
That's the beauty of an art wall: It can grow and change with your tastes. If you have high ceilings, you can add more art on top; if your gallery wall is in a hallway, you can keep going with it right on down the hall. And, of course, you can always swap out the photos or artwork with new picks as long as they fit in the current frames. The possibilities are endless. Have fun with it!