Lay out the quilt on the floor, smoothing as you go so it is straight and flat, but do not stretch...just let it lay flat. Now take your border strip and lay it down the CENTER of the quilt, top to bottom, just smoothing it out. DO NOT STRETCH, just smooth, smooth. Trim it off at the bottom of the end of the quilt. I lay my quilt out on carpet, so I also like to anchor the beginning end of the borders with a couple of long pins stuck into the carpet. It keeps that one border end from creeping as I smooth. Cut the second strip the same way.
It is really important to cut your border strips straight across, or you can compound the problem if that angle is off from 45 degrees. If the angle is too wide, your next border will flare up at the corners. If it is too narrow, it is going to pull the top/bottom border down and your side border will be too short. Keep those angles square when you trim!
If you are doing a square quilt with 4 cornerstones in the corners, you can cut all 4 pieces at the same time.
If you are doing a rectangular quilt with 4 cornerstones, also cut your top and bottom borders across the width of the quilt, measuring through the center before sewing on the side borders.
This has ALWAYS worked for me, because when you are dealing with measuring tapes, they dont drape and lay the way your fabric does....your border fabric is your 'measuring tape' and as you smooth it across the center of the quilt top it will lay the same way as the quilt center with no tension on it. (copied from Bonnie Hunter's Border Hints post)
Bonnie's other border tips and hints can be found here