a friend of mine does breathholding. He lies on his back, relaxing, and takes some deap breaths and holds his breath for as long as he can. Then he breaths calmly for three minutes and repeats. He dos this four times or so. He says this is what good freedivers do before diving. Myself, I am often anxious to get in the water, so I skip his warmup. However I often experience that it is not until after 5 or 10 initial dives that I feel warmed up and can go deeper and get that nice relaxed feeling. I think with his warmup, you already reach that state before you go in. I guess the freediving forums probably have loads on this subject.