There is a product made just for this purpose. It's called a teflon pillow and it is slipped inside your garment before pressing. It keeps the front and back layers apart when pressing so you do not get the lines. They come in various sizes. Here is one source for them. Scroll down for Teflon Pillows: Equipment ID - Stahls' Accessories
A forum community dedicated to T-shirt merchants, professionals and enthusiasts. Come join the discussion about graphics, commerce, dyes, prints, collections, guides, displays, styles, scales, equipment, accessories, reviews, classifieds, and more!