Lots of great information has already been mentioned here. One other piece of advice: I read somewhere not to be afraid to add some colors that you don't feel safe with, such as cheddar yellow, gastrointestinal tract green, chili pepper red, etc.
I have a quilting friend who routinely chides me because I generally shy away from buying a "matchy, matchy" line of coordinating fabrics. I'm always on the look-out for something a little more out of line with normal. When we shop together or sew together, she looks at my fabrics and shakes her head in doubt, then when the quilt is done she shakes her head in amazement at how beautiful it is (her words not mine).
Go with your gut about what you like, then add a little adventure!