I've had good luck with the blue PNP sheets but the copper definitely has to have "teeth".
Scrub the copper (or other metal if you're labeling with it) with an old toothbrush and Comet or other abrasive cleanser. Rinse with clean water, wipe the water off with a paper towel and never touch it again ! I put about 20 sheets of old letter paper on a flat surface, under the pcb and one sheet on top of the blue sheet. Press with a hot iron (highest setting) without moving for about 10 - 20 seconds, then move to a few different orientations (probably helps to make it "hit" all areas).
Do not move the iron while applying more than light pressure as you may smear or distort the heated toner below.
Cool under running water before peeling away the sheet. The last board I did was about a month ago and had 4 or 5 tiny SSOP packages on it. Not perfect but had 98% continuity before touchup.
This would probably not work on any thicker material (1 oz. pcb is 1.4 mils thick) without a preheat or baking as mentioned.