At the top of the pleat, in the last 1/2-inch or so of material that will end up being encased in the waistband, simply take 2 or 3 stitches very close together and tight (like whip-stitching, but with each stitch almost on top of the last one). This is plenty to hold it together while you continue work. Once the waistband is sewn on, it locks all the pleat stitching in place for good.