I took a class on this very thing last week and here's what the gal did. She got some see through tissue paper and drew a design on it. She put her quilt up on a design wall (tape it to your wall if you don't have a design wall because then you can step back and look at it). Then she pinned the tissue, with the design printed on it, to a block on her quilt. Because the tissue is thin, and you can see through it, you can see what the design will look like on the block. She did a different design on each piece of tissue and then pinned it to a block until she found the design she liked the best.
When you stand back and look at the different designs, it's pretty amazing. There's always one that catches your eye and you say "that's it". And there's always some that make you say YUK!