My favorite secret weapon for hemming knit fabrics is my twin needle. It creates two rows of straight stitches on the front of the fabric, but one row of zig zag stitches on the back which allows the seam to stretch with the fabric. Most sewing machines can take a twin needle without any extra attachments. Domenica from Easy Sewing for Beginners shares a video tutorial showing how to use one. She includes how to set up your machine to use the needle, and also what the numbers on the needle packaging mean. Go to her blog to watch her video.
[photo from Easy Sewing for Beginners]