The biggest things I do to prevent the dreaded flare is to stay stitch about 1/8 of an inch around the whole inner quilt top before adding the borders (especially if they have a lot of piecing). Then I measure the lengths I need and cut my border to that, pin at both ends, then pin the middle, then add additional pins if needed. (I really don't pin every few inches). Then, I get my trusty old walking foot out and use it to make those long seams. This helps to feed both the top and bottom layers of fabric through at the same speed. If you don't use a walking foot, the layer of fabric on the bottom feeds slightly faster than the top layer as the top layer is basically just resting on the bottom layer and been pulled along by it. When I have not used my walking foot, I have found that although I started with the same length, I may have over an inch difference by the time I get to the bottom.