Another important thing to use when you sew with knits is the right needles. For stretch fabrics you want to use ball point needles instead of universal needles.
Ball point needles are used for stretch fabrics for two reasons:
1 - the ball point of the needle glides through the closely woven loops of knit fabrics and therefore does not leave unwanted holes in your garment
2 - the stitch of a ball point needle is less straight allowing a bit more give and stretch for your fabric along the stitch line.