Print or Solid?

This seems like it should be easy enough to figure out, but how do you decide what fabrics to put together? If you put all prints together, won't it look 'too busy', especially in larger pieces? Do you pick a solid to be the 2nd focus, or just a complimentary color?
And, if you are using a calico or a small print in your focus fabric, do you go with a similar print in your other colors in the quilt or is there a rule like, 1 large print to a couple smaller coordinating prints and a solid, or is there a ratio involved? I understand the color wheel, it's just putting prints together that stumps me.... I looked thru the charm packs and jelly rolls at the stores for ideas, but sometimes they have me more confused than before I started....

Any help would be appreciated!

Kathy