Whenever I MILD I don't use a set mantra. I use many sayings, and sometimes I'll use a saying I haven't even thought of before it'll just come to me in the moment. My favorite sayings are directed to my subconscious such as: "My subconscious will help me to lucid dream" "My subconscious will present me with many dream signs" "My subconscious enjoys lucid dreaming" etc. The first night I used these I had 3 lucids. Try different things to see what works for you. And as Supernova already stated, the key is to have the intention of lucid dreaming the very last thing on your mind before sleeping.