When poaching, it's crucial to use high-quality, recently laid eggs. As eggs age, the proteins within the albumen break down, becoming more watery and less gelatinous.
As a not-so-fresh egg hits the almost-boiling water, the white disperses, sending the albumen in all directions and exposing the yolk to direct heat, resulting in uneven cooking.
As a rule of thumb, for a single poached egg, a medium sized pan will do, with at least four inches of water. An egg can sink and flatten in a pan that's too small.
As an egg cooks, the protein within the whites begins to break down and solidify, which relies on significant heat. However, increased acidity can have the same effect.
Dr. Robert Keyzers at the Victoria University of Wellington says that adding drops of vinegar to the boiling water can "...help the poached egg hold its shape better."
Breaking an egg into a bowl before poaching will give you the control you need, allowing you to slide the egg under the water's surface without fear it will break.