First, this may sound really silly, but I always fall asleep easier when I'm not trying hard to sleep. Letting my mind wander always helps.
What do you do during the day? If it is something monotonous such as working at a computer or something, I'd understand how you'd have trouble sleeping.
Exercise vigorously, or do manual labor of whatever kind you can manage! The tiredest I have ever been and the best I have ever slept is when I was running in the morning and evening, giving myself a good hard workout, and chasing after kids and doing chores all day in between. Cut all the caffeine and unneeded sugar out of your diet...I know you said you tried temperature variations, but I always get sleepy when the room is a tiny bit warmer than it needs to be...put on a movie you've seen a million times and turn the volume way down low...read a book that is a little bit heavy reading and read it laying down...never fails to make my eyes droop. :) I have heard that foods like turkey and milk are natural sleep inducers...maybe make yourself a hearty sandwich and eat it right before bed? Full tummy also makes me sleepy. Of course I am 19 weeks pregnant right now so i am ALWAYS sleepy...but I know that sleepless feeling. Good luck! Hope you get some rest!!