I can't sleep on my back either. It's not comfortable for me to be on my back more then... I don't know... 15 minutes.

I best do my relaxt thing on my back, I'm totally relaxed, heart start to beat harder, breath is deeper, I start to see random scenes and hear voices in my head, focusing on my "lucid dream feeling", I feel succes, but - I'm sick of beeing on my back (of that position) and I have to get on my side and everything is gone