Well, it is said that sleeping on your back is probably the best position for WILD’s; but i, and many others, cannot feel relaxed enough in that position. So, generally, any position where you can feel totally relaxed, and your body can fall asleep without causing you any inconveniance later on in the induction, is a good enough position.
As for getting to the HI stage, all i can tell you is to try to keep your (logical) mind alert; counting to 100 while saying “I’m dreaming” and back to 1 and so forth, is a good way. Not too alert that you can’t sleep, but not not alert enough that you do fall asleep.
As for getting through the HI’s, i am struggling with that myself. 
This thread could help.