|Knit fabrics don't behave when sewing across the knit. They like to pucker and shift, usually when you're trying to get a nice flat hem on a t-shirt. Using a twin needle helps, as well as the finger behind the presser foot technique, I personally use steam a seam to stick it all together before I sew.
I thought, why not go with the flow on this? Instead of fighting the knit, use it's puckering abilities to add texture to the knit. I cut a t-shirt out of a patterned knit, any basic t-shirt pattern will do, then took the front, set up my sewing machine for twin needle and proceeded to abuse the fabric, stretching it, folding in ridges with my thumb, going loop the loop, around in circles. Using the twin needle with the tension a bit out of whack makes it ridge up a little bit between the stitches, adding even more dimension.
I then put the top together, using serger on the seams, and a wide 3 needle hem on the sleeves and bottom. I serged the neck, then folded it over and finished with twin needle, making the fabric behave in that instance so the neck wouldn't stretch out. My daughter was estatic, textured distressed hippie sort of things are all the rage.
So, don't fight the stretch in knits, take advantage of it and change the nature of the fabric.