If anything, your body and mind should be confortable in order to go into lucid dreams with ease.
Physical confort is pretty straight forward. Relaxations techniques are a great aid if he have some tension in our body. Mental confort can be tricky, so I’ll suggest the usual “I’am confortable” affirmation.
Hey… if the shoe fits, you won’t feel it! 
Something I’ve always wondered about people using the WILD technique. Is it done lying on your back? Because when I go to sleep at night, the only way I ever seem to be truly comfortable is lying on my side in a semi-fetal position.
I can lie on my back for a long time, but I always start to feel like it’d be way more comfortable on my side, so I roll over and pass out.
In my signature you can find WILD FAQ so look there if you want to know more about WILD
As for position, you can lay in any position you want as long as your comfortable, but many WILDers lay on their back so they don’t loose their consciousness that easily.