Whenever I have digital photos of fabric to work with, which is almost always the case when you can find fabrics online, I use my computer's paint program to compare how the fabrics look together. You can then also resize the pics to scale, paste and cut and design many sewing possibilities. I also purchased a program called Kaleidoscope Kreator 3 and have found that very helpful for deciding if a fabric will work well for One Block Wonders. Both of these methods have saved me from buying fabrics that wouldn't work well many times over - and they are tremendous fun to play with.
Never thought I'd be interested in purchasing a quilt kit, since I enjoy designing and cutting with fabric. But i fell in love with a series of Kaffe Fasset fabrics and couldn't find them for sale, even with online searches. Finally, on my favorite online auction site (don't know if we are allowed to mention the name here on the board) I came across a seller (jordansoforegon) who had done up the fabrics in the nicest log cabin squares as kits.
So I used my computer's Paint program to decide on coordinating fabric for borders and come up with a final design. Though not a perfect representation, this technique lets me see what a quilt might someday look like. I'll paste a picture below.
Many Fassetted Log Cabin Quilt (arr, arr)