I got this tip from Quiltville:
Lay your quilt out. Lay the border fabric on top in the center (either vertically or horizontally, whichever you are doing first). Smooth the fabric out, then cut it the same size. I pin at a few intervals along the way so it doesn't move, then bring my cutter and small cutting mat over and trim the strips. Match the ends and center points and pin before you sew. Press your border seams and repeat for the other borders.
The reason she does it this way is because measuring tapes can stretch, but if you smooth the fabric out the same as the quilt, it doesn't. I have done many quilts this way and never have problems with wavy borders. Give it a try!