I try to buy my backing when I buy my fabric to match the one I like best However I am bad about buying fabrics I like and have no idea what I am going to do with it. Then I try to find something that looks like it was made to go with it. I have always been lucky to find something. I have only used solids once in a baby quilt. I have made two tops with scraps from the main top and loved the way it turned out. On that one I had to find solids that matched colors in the prints for the one I used for the back.

I like picking a busy fabric for the back that has at least some of the colors from the front incorporated. Busy fabrics help hide quilting mistakes. Sometimes I will pick the reverse of the front. I did one quilt that was pink & white with little appliqued doll dresses on the front. On the back, I found a multiethnic matruska doll fabric with a black background. It looked absolutely beautiful & was easy to seam down the middle due to the repeating pattern.

The quilt I designed myself has a seascape scene, so on the back I am seaming together 3 fabrics -- RK Ocean, RK Atmosphere & a solid teal fabric to match the teal line that runs down the left margin of both. It mimics the front both in color scheme & with the bubbles since I am appliqueing bubbles on to the front. I always think it's nice when you can carry a theme to the back of a quilt, but that's just my very humble opinion.

It might be fun to pick a bright colored fabric for your back since you did black and white on the front.

If it is a backing for a wall hanging then I pick something 'ugly' from my stash since no one is going to see it anyway, or even piece the backing. If it is a bed quilt, lap quilt, placemats where the back will be seen then I try to use something pretty. I also take into account how I plan on quilting it, colour of thread, etc when choosing my backing

IF the back will be seen, like a throw or baby quilt, I try and match the color to something on the top. If it's a bed quilt, where the back won't be seen much, I just use a good quality muslin.

I make my top and then go to my stash and pick out one of the fabrics I have picked for a backing fabric. With black and white you could use anything on the back, just something you will like to see the quilting on. LIke someone said, the printed backs don't show little goof ups as easily, then use a pretty binding. It is your quilt make it what you like.

I like a backing to complement the top. If the top is wild and crazy, then the back is quiet and composed.

I did see a black, white and red quilt where the maker had used black and dark gray print fat quarters for the backing. It was beautiful and fit my 'quiet and composed' criteria for the top.

I think the backing should speak to the front. Think of the corner of the quilt folding up onto the quilt. Would they go together? It is one quilt. I buy my backing fabric after the top is finished. You could piece fabric from the front and put it in the backing fabric as well.

Well I am a very traditional quilter I think. I like more traditional fabrics. Many have called them old fashioned looking but that's just my style. I like a full piece back or if it's not wide enough, pieced from one fabric. Small prints that compliment the front. I do a LOT of white on white backings. Might be because all of the quilts I grew up with had solid white muslin backs. It's just what I'm used to and what brings me comfort. Of course all the quilts I make are functional; made to be used over and over. I think the backing is just like the front and batting - it's all a matter of personal preference/taste.

I purchase my backing after finishing the top. It is amazing how you select a different colour when it is finished. Not many extra wide outlets around me so normal width but I do try to get lengths when they are on sale so I have a choice

Like DottyMo I always wait until the top is done before selecting my backing - opinions can change dramatically over the course of making the top!
I usually use extra-wide, but will sometimes piece 'normal' width together - I've not done a pieced backing incorporating blocks, etc. yet.
When selecting colours I always consider if it 'goes' with the top - I personally wouldn't want anything that would look 'wrong' if the corner was flipped and you could see the top and the back together. I usually go with some sort of patterned backing, but that can be anything from a vibrant print to a gentle tone-on-tone. As others have said, consider the quilting that will be showing - the quilting is highlighted far more on a solid (this can be good or bad! LOL)
Also consider the colour of thread you intend to use with both the top and the back - again, variegated tends to blend more and a solid will usually stand out. I know a lot of people use neutrals (taupe, grey, etc) but I like to pick up a colour/colours in the fabrics used.
It really does come down to personal choice, but I'm finding it very interesting to hear how other approach this - people come up with things I've never thought of.