Something that has helped me is developing a meditate-to-sleep routine. It takes time and practice and can be improved upon infinitely. I have stored away subjects and certain interactions/experiences/memories that I find very relaxing. I've practiced being able to recall these things and combined with very focused deep breathing and other basic meditation techniques I can usually drift off relatively well. Again, it takes practice. It's hard to push away intrusive thoughts when surrounded by unfamiliar noises and circumstances (such as lying in a sleeping bag on a pad in a tent far away from your bedroom). I take it as seriously as riding to get in shape for a tour.