I love gallery walls. I think they're great for filling up that big expanse above your couch or for drawing attention to a little nook. And you don't need big, important pieces of art - small ones are great. A mix of sizes is great. You can be neat and orderly with similar prints in matching frames, be really eclectic with different mediums and a wide variety of frames or something in the middle. Maybe similar frames but not exactly the same. Not a completely random set-up, but not overly symmetrical.
What I also love about a gallery wall is that they can evolve over time. Maybe you only have 3-5 pictures now. But then you're at Eastern Market and see a lovely little watercolor. You're at Pottery Barn and they have a pretty little salad plate. You take a pottery class and put your vase on a tiny floating shelf. Your best friend took a gorgeous picture while the two of you are on vacation so you blow it up and have it framed. Your grandmother hands down a silk scarf that's too threadbare to wear but still ridiculously lovely. Eventually you end up with a wall full of special items and your gallery wall had turned into a memory wall. What can be better than that?
Or maybe you have a few large mirrors. Or a collection of salvaged items (or, like in one picture below, a bunch of plaster medallions). A gallery wall doesn't have to pictures at all.
When I did mine, I found it useful to clear out everything from the middle of the room and lay out everything on the floor. I moved the pieces around and pulled stuff from other rooms until I had a good set-up. Then I took pictures of it all and did a rough sketch so I could remember what went where since I had to stack everything up so I could move the couch and get my ladder near the wall.
Supplies are important. I used a laser level since I wanted certain pieces to line up - but not necessarily ones right next to each other. A pencil with a good eraser so I could mark spots - and erase and change them. A hammer and nails. Picture hooks for the heavier pieces. A ruler or measuring tape so you can space things out evenly. I did 3" in between my pictures, but wish I had made the space only 2". But you have to figure out what's right for your room. Generallybetween 1-3" is good.
And patience. Patience might be the most important supply. Make sure you have a lot of it! I had to rehang almost every piece 3-4 times, even with my laser level, measuring tape and gobs of pencil marks. So frustrating, but so worth it in the end.