From Wikipedia:
"To prevent dispersion of the white of the egg, it can be strained before-hand, removing the parts of the white that are likely to disperse and creating a perfect, compact poached egg every time, with very little effort and without resorting to other risky methods such as vinegar.
Another method states that a small amount of vinegar may be added to the boiling water. However, this technique is risky, as one may add too much vinegar into the water, resulting in a dry egg and an acidic taste. Stirring the water vigorously to create a vortex may also reduce dispersion."