It's pretty much unavoidable that some pieces of cauliflower fall off the cauliflower head as you slice. However, I find the best way to cut the cauliflower is to cut the bottom stem end off first so there is a flat base on one side of the cauliflower head. Place it flats side down on a cutting board. I slice the head in half from top to bottom first, then in thick slices (also from top to bottom) until the whole head is cut.