make sure to pick a stitch that is made for knits otherwise the fabric stretches, the thread doesnt and it goes POP as happens with my HP scarf all the time

knit stitches have more thread in them - the straight stitch does something like forward once, back once and forward twice or somethign weird like that
its not necessary but you can also use the technique of hand winding the bobbin with wooly nylon. its a thread that has some stretch built into it. polyarn has the same feature but is made of polyester so it has a better tolerance for heat (ie ironing or the dryer)
dont forget to reduce the pressure on the foot when you sew it! unless you want the crazy waves : )