I was making this skirt a few days ago and as I did something I thought "I wonder if other people do this too?" You might think it's overkill, but I do still think it's a good idea.
So I'm sure you already know to use non-roll elastic for waistbands. Right? Don't you hate when a waistband gets all wonky inside the casing? Non-roll and this step I'm explaining here both make that impossible. Another reason to love handmade. :)
So what I usually do is "stitch in the ditch" along the side seams. If they are pants, you don't need to do it to all four seam, the two side seams should be plenty good.
Start a few stitches down from the top and then backstitch to the top and then back down to the bottom of the casing, making sure to backstitch again. Pull the threads to the inside and tie them off. Easy peasy!
When you hold the seam like this, you can see that the stitches are very close to the seam. If you can get them right on top, even better. But it can be a little tough to be exact since you have to also pull the elastic/fabric flat while you are stitching. It's only noticeable below because I am folding it backwards, which you don't do when it's being worn. Right?
When the piece lays flat it's virtually invisible.
Now you've just secured the elastic, which I think prolongs the life of the garment.