I've been free-diving sice I was about 10 years old. I never practiced in a pool, just started in the sea, not to far from my house.
The mammalian reflex, like mentioned, is not something you need to fight, it helps. You probobly ment the breathing reflex (whatever it's name in english). Like mentioned- the best way is to take 2 deep breathes, that refresh the lungs and take out CO2 and than a third one, very deep, and dive. remeber though not to take more than 2 breaths before the dive, becouse it may couse "Shalow water loss on concienceness" (again, dont know the reall english name)that is coused by to low CO2 levels.
You need to practice relaxing, and controling your breath, which are things you can actualy do out of the water. You also have to learn how to use the minimal amount of energy possible to prolong your stay (I can hold my breath while not moving for about 4.5 minuets, but if I swim I can stay less than 2 minuets).
One last thing- For a precoution, if you plan to do deep freedives, alwais have a buddy that can rescue you if you lose conscisness. All it takes to rescue usualy is diving 3 meters and bringing you to the surface. Once you get to the surface you'll get ok quite soon.