If I have solid or "reads solid" sashing, I will use the same material as the first border since my blocks float and I like the look. I also frequently use a flange of black, white (or some color that may be in the fabrics, but not dominant) so the flange can stop the pattern. I then use the guidelines that Paper Princess outlined:
1) The final quilt has to be a certain size, so I add borders to make it that size
2) I only have a limited amount of border fabric left, so that's how big the border is.
3) The border fabric is a print, and I want a particular portion of the print to show.
4) I use the scientific audition method. I sew on the narrow inner border, lay the outer border out & move the quilt back & forth.

There is one exception -- sometimes the quilt tells me to stop, it's done. I have been wrestling with a Halloween because it is telling me to stop at a wall hanging (I have no space for one) and I want a lap quilt. Sigh -- hate it when that happens. :mad: