I don’t think finding the perfect mantra is the key to MILD. About 90% of all my LDs have been from MILD and what I learned so far is that you got to train your mind to remind itself of its intention. After you do MILD for a while, usually your thoughts begin to trail off as you go to sleep. You have to try to catch yourself at least a few times and remind yourself of your intention. I usually try this as many times as possible and I eventually miss it one time and I end up spontaneously remembering my intention in a LD later (hopefully).
That’s my variation, so I guess MILD can sort of be done in different ways.