I had similar problems with my heated glass bed on the Wilson II. I tried the glue-stick supplied by Marty but clean-up after a few prints was messy (the glue stick leaves a thick layer which must be scraped off).
I discovered a better approach. I thin elmer's white glue and dab on the cooled glass surface with a wet paper towel. If the glue is very thin and the paper towel is wet, it will produce a thin watery pattern kinda like the sponge-paint textured walls people used to paint. If you heat the bed this will dry rapidly and leave behind a VERY thin film and I've found my PLA sticks to it extremeley well. if the film gets beat-up after a few prints, just let the glass cool, dab some wet paper towel on it and reset the film -- the white glue is water soluable.
As AndreQ suggested, make sure your z-height offset is proper so the first layer is slightly squished onto the glass from the nozzle. That also helps a lot. Not enough squish and it won't adhere as well. Too much squish and you'll block the nozzle -- you'll know if that happens because you can hear the PLA slipping on the extruder gears because it's getting blocked.