|Painters' tape is the blue masking tape used for painting your house. It is easier to remove than the traditional kind.
Like a lot of good tips, I made this discovery by accident. I was trying to hem a knit fabric with wide ribs and I couldn't get it to lay flat. I was concerned that the hem would look bunchy or stretched out instead of smooth. So taped the area to the immediate right of my stitching line and sewed the hem with a 4 mm straight stitch. Perfect! I tried this method on another t-shirt and again, it looked fine. The tape acts as a temporary stabilizer and gives an edge to guide your stitching (I don't sew through the tape, but just next to it).
Of course, you will want to test your fabric to make sure the tape won't mar it. To avoid problems, I recommend sewing right after applying the tape and removing the tape before you press the garment.