The simplest way to eliminate transfer lines from sublimation is to make the paper larger than the heat press. This will illuminate the paper causing the lines. One thing to note, you can still have issues with the edge of the heat press causing lines, but this is normally due to having way too much pressure.
You want your pressure to be medium for garment transfers with sublimation and that is between 40 to 50 psi (a four or five readout on most presses). One other tip is to thread the garment to give it less depth to get pressure lines.