I hold the fabrics next to each other and decide if I like how they look together. It's usually not too hard to find two or three that I like together, but then finding four, five, and six that all 'go' gets progressively more difficult. After six or so, I'd say the more different fabrics, the less they have to match, and it gets easier again.
How this translates into advice is that I would try to make a quilt that only requires three different fabrics, or I would try a scrappy quilt.
Tonals add some variety over solids and can be easier to match than prints. You might have one print fabric with several colors in it, and then buy tonal fabrics that match some of those colors. (A couple other people have already said similar, so I'm just agreeing that it works for me, too).
Don't be afraid to bring a piece of fabric to the store with you to match - for a while I carried around some particularly difficult fabrics with me in my coat pocket. I have one pretty blue fabric that does not match any other shade of blue known to human kind and will have to match it up with black, because black matches everything.