This one might seem obvious, but I figure it's worth stating anyway: if you do something once, do it again, then do it again.
So, if you use a colour, or fabric, once, repeat it. And by extension, if you combine two colours the more times you repeat the combination, the better it will work.
An example: when I started Two Hungry Birds I wasn't sure about this brown striped fabric. I thought it might be too strong a contrast, might jar.
But I decided to stick with it but made sure that I kept looking at how often it was occurring as the quilt grew. I think if you kept the level of repetition right then it acquires a kind of internal logic (if that makes sense), so that almost anything works if you do it just often enough and no more.
When I was making this quilt I had no red at all in my initial fabric pull.
The child for whom it was made requested the red birds in no uncertain terms, so I included it, and then added in the other fabrics with red. Use it once, use it again.
I tend to follow the same rule when I'm piecing. So, if I slice a strip up and add a little bit of something else, then I'll keep on doing it. If I add a star, I'll probably add another one somewhere along the line. Annie's quilt is a good example of this. The first time I put a little slice of red or orange into the vertical turquoise strips it was just a pragmatic way to join two bits and make them long enough, but I went on doing it the whole way through the quilt.
The same thing applies with the stars, the little log cabin blocks, the blunt nosed triangles.