I've always found quilting borders a challenge too. The one thing about quilting borders that I learned at a machine quilting course was not to try and quilt them vertically using straight lines with your walking foot as this tends to stretch them. Our instructor had us zigzagging down the length, turning the quilt around and zigzagging back up which resulted in a pleasing design of rectangles or squares depending on how far apart you had made your marking points to change direction. I haven't tried this technique on plain fabric but it worked well for my brightly coloured ones.