If you mean piecing short strips when you need a longer one, a lot of people like to use a diagonal seam which would involve putting them at right angles and sewing a diagonal seam. What I use depends on the print of the fabric-sometimes I piece diagonally and sometimes straight.
If you mean borders that are mitered at the corners of the quilt, then you don't do the mitering until after you've sewn them to the sides of the quilt. If this is what you mean, I believe there's a good tutorial in the tutorials section of the board.