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.