Alex Anderson told me a few yrs ago that sometimes the problem is a lot of quilting in the middle then less quilting on the edges or border. I know you don't have a border....in that case, I definately stay stitch the edges (no stretching when doing it either) Then measure thru center of quilt both ways and add binding.
The quilt I asked Alex about has to have the border taken off and start over from there, with lots of hand quilting. I'm just sick about that but I will eventually do it.
A quick fix for you is to put a rod pocket on the bottom and just get a dowel to flatten it out while it's hanging.

What really helps me is to snug up the binding as I sew it on, as it takes up the looseness on the edges that aren't quilted. So far it works ok for me.

I have to disagree - and I apologize for using this particular posting to quote, but if you measure the center and make the borders that length all you are doing is stretching or easing the borders to fit that measurement. If the edges of your quilt are not the same as the center, your quilt is not square, and stretching or easing will only compound the problem -visually. You might ease the border, but then all you'll have is the center puckered into a border. The border might be straight - you've just moved the problem in to the center. You have to correct the problem of why the edge of quilt is not right. Do you have to just trim a bit? Or take a bit more of a seam in some of the pieces. But if the quilt doesn't lay flat before the borders added, it won't lay flat when the border is on.

The best way is to measure through the center of the quilt for the length and width, as blocks may vary a little, this way you get the edge you want.

The reason for measuring through the middle of the quilt and cutting the borders to those measurements is so that excess border is not sewn onto the quilt. The most common cause of wavy edges is sewing a border on without measuring first because chances are high that either the border will stretch as it is sewn, or the feed dogs will feed more border than quilt with each stitch. This also takes care of minor differences in measurement in the quilt top itself (e.g., one side slightly longer than the other). Trying to solve large discrepancies in this way won't work for the reasons you provide.

I'll bet you're not using the "straight grain" of your fabric as the edge of your quilt. If you're cutting your fabric "WOF" (crossgrain) that will result in waves. A little more time consuming to cut on straight grain, but the results are well worth the effort.

You could also try wet blocking or dry blocking the quilt.

Question for all of you... if you have a quilt with no borders and will quilt it on a sit down machine do you all think this would help putting a stay stitch in to keep it from getting wavy. I'm almost done with a Farmer's Market and I'm fretting about quilting it.

Thank you SO MUCH each and everyone of you for responding. I'm going to copy and print all of the suggestions and make a notebook for tips on helping me do my LA quilting. You girls are the greatest for taking time to answer this and it is much appreciated:) This board rocks:):) I don't exactly know WHY it happened....but, I'll try to take preventitive measures for the next quilt. Thanks again, debby:)

Cutting borders on the lengthwise grain works better if you have enough fabric. The lengthwise grain stretches less than the crosswise grain. I also run a line of stitching around the whole edge of my quilt top so that even if it is handled a lot before it gets quilted it won't stretch.