Well this is what I'm doing today. The printed side is touching the PCB, I always make sure of that, but the final result is rarely as good as it can be when doing bigger boards. I'm using two pieces of plexi glass and they are as fare as I can tell perfectly straight. I also often place something heavy on top during exposure, but still no good. Usually things work out okay, and the parts that are good (usually 90-100%) are perfectly good, but I would like consistent results every time. A saw a guy using 3 in 1 oil in a youtube video, he said that this would make a really professional result. I don't really know what type of oil this is exactly though. I am making double sided boards though, single sided boards give more consistent results (not sure why).