A floral, two Hawaiian prints, a thirties wannabe, a plaid, and other various assorted fabric nightmares. All these fabrics were thrifted from old clothing or scraps.
Hmmmm...not too bad, eh? Those prints got quieter as more were added. In mixing patterns, I find that varying scales and textures is a must. Prints that are similar--like two similar scale florals--will clash. But a floral and a check? Wonderful. A stripe and spotty thing? Fabulous. Vary your texture and scale! I know it can be traumatic your first time, but it takes a leap of faith. Just do it. Don't freak out over one ugly block. Twenty ugly blocks are not so ugly any more.
The bigger picture. Hey! Where did all those clashy fabrics go? Where did those horrible eye-assaulting wild things go? (Okay...if you can still see them, I don't want to know. Work with me here, huh?) Always keep the bigger picture in mind and just go for it. Have faith in the bigger picture.
Color is almost a secondary consideration in my mind. I usually try to use one dominant color more than all the other colors in the quilt to tie everything together. Don't use equal amounts of all colors--they tend to cancel each other out. When making a two-color quilt, for instance, use 2/3 of your dominant color and 1/3 of your accent color. When using more colors, divide them up accordingly, always using more of your dominant. An easy way to do this is make your dominant color your main border or sashing color. A strong border will help to corral all the wild little print ponies and keep them from running amok. By "strong" border, I mean a texture or tiny print with only one or two colors that reads like a texture.
I almost never use solids. They attract the eye too much. Your eye falls into a quilt black hole and won't move around the quilt any more! Many beginners use solids because they think it will blend in and viewers will only notice the print. Actually, the opposite is true. The solid fabric shape sticks out like a sore thumb and you don't notice the print because it's shape is broken up by pattern. The only time I use a solid is if that's what I'm trying to accomplish--a strong, clear shape that makes everything else background noise.
Because if it doesn't happen in black and white, it's not gonna happen in color. Trust me on this one.