You have received many excellent suggestions...stabilizer, needles, presser foot, etc. One other thing you can try...Go around the item twice; first with a less dense and narrower zig zag, and then with the satin stitch zig zag. I used to do this when appliqueing my kids clothes years ago. Something about the first zig zag made the second time around look better. I heard someone say that on a sewing program once too, so decided I wasn't just me that needed to do that to get a nice satin stitch.