What’s the best way to sew seams in knit fabric to keep the seams from popping?
Ideally, the best seam for knits is made with a serger–either a four-thread or three-thread stitch, so that there is inherent stretch in the seam to allow for wearing movement. If you don’t have a serger, use a narrow zigzag stitch along the seamline and trim off the excess seam allowance width to 1/4″. If you must use a straight stitch, stretch the seam slightly as you sew and stitch a second line close to the first before trimming the seam allowances to 1/4″.
Knit seams tend to curl if left with a 5/8″ seam allowance; trimming helps reduce that problem and makes a flatter, less obtrusive seam.
In areas where there can be lots of stress, like a pant crotch seam, a combination of conventional and serging stitching helps to reinforce the area.
For more information, see Guideline 4.215 Sewing With Knits.