Fill the bobbin thread with wooly nylon. Use a twin needle to sew using regular (not wooly nylon) thread.The Secret to Hemming Knit Fabric (it's all in what type of thread you use) - Rae Gun Ramblings
How to sew and use packing cubes - Melly Sews
Fix for a top that is too tight around the bottom. **Great to not have a banded bottom on a top. By spreading the side seams magically removes the band.