For me, the only thing leading to consistency is motivation, which is in itself not consistent.
Level of difficulty with reaching the OBE state itself fluctuates as well, influencing level of motivation needed at the different times to be succesful.
This interacting factors make it very difficult to predict success on any given night, as well as confusing as to what makes the OBE happen.
I hope one day very little motivation will be required. That would solve it. I still find myself at times to have to be heavily motivated however.
Often I don't even recognize it, and spend nite after nite trying to go the "easy" route without success, while staying hours awake at nite is what is required.