For borders, it's important to measure through the middle of the quilt, cut the borders to that length, then divide and pin the border to the quilt before starting to sew. The most common reason for wavy borders is that the quilter has simply taken a length of fabric, sewed it to the quilt, then cut it to fit.
Starching fabric stabilizes it so it is much less likely to stretch out of shape or distort as you work with it. Starching before even cutting your pieces helps ensure accurate cuts. Spray starching as you assemble will help keep blocks flat and straight.