Sometimes when making gowns that have a lot of gathering at the underbust seam (say, a kimono bodice paired with a straight cut skirt or full circle skirt), instead of serging elastic into the seam like I normally do, I'll make a casing with twill tape, and then add my elastic into the casing.
This is just a great way to even out your gathers while keeping the seam nice and stretchy (you can even easily adjust the size of the gown, this way).
This is what the inside of the gown looks like with the casing over the seam. I stitched it close to the edges on both sides.
Here's where I left a gap to thread the elastic through. First cut the elastic in the length you need (the wearers under-bust measurement), and attach a large safety pin to the end. Then, thread the elastic through the casing, and when you get to the end, overlap the ends, and then stitch then together.
Here's how the casing looks from the outside.