Another thing that REALLY helps, especially with stretchy knits is lengthening your stitch length, my usualy setting is 2.5, I use 3.5 for topstitching knits. I've also found that I need a super stretchy hem with my destructo boy- he can blow out the best hems on tshirts, lol. I have to use a wide long zigzag for his shirts- it's about the only thing that works.