The down and Dirty Way:
Run a few zig zag stitch lines over the gathers, its not fancy but I bet you it won't move.
The Fancy Way:
Knit Fusible Tape applied to seams.
My Favorite Way:
Run a piece under the serger, DONE!
Now, I'm not saying my way is right, I'm saying my way is fast, effective and easily accessible at your nearest hobby and craft store.
I use elastic mainly on side seam shirring because side seams take a beating with every move you make
they pull apart and come together and on my hips, God bless them!
I need something that can stretch and stabilize at the same time.
Plus you know I don't like zippers on knit dresses unless I absolutely have too.
I can wiggle into my dresses and my seam stitches don't pop.
As for what you are seeing here, you'll find out on Twiluv 58