Measure the quilt before you cut the borders.
Measure length, measure the middle, and measure the sides of your quilt. Average out the measurement and use that measure to cut the length of your borders. Pin the borders on starting at the middle and then the ends, and then finish pinning easing in fabric fullness if any.
Then measure across the top of your quilt, across the middle, and across the bottom, including the side borders already put on. Average your measurements and use that number to cut your top and bottom borders. Again, pin border on center first, ends next and ease in extra fullness if any.
This method work very well and borders should lay pretty flat.