It takes a lot of practice of balancing on the edge of waking and sleep. The more you do it and experiment with degrees of letting go vs. holding on, the better you get at recognizing the subtle signs that sleep is approaching. I find that I need to work almost solely on relaxation and letting go to have a hope of falling asleep when trying to WILD. I'm still working on figuring it out, but I feel like every time I get at least a small fraction closer to more complete understanding and learning that "feeling" of approaching sleep and being able to reproduce it upon demand.