1. Do not let the water boil. You want to maintain the temperature at just at the edge of a simmer (around 180°F), where the bubbles just occasionally come up to the surface.
2. Do not stir the shells. Stirring will muddy up the stock.
3. Skim the foam. As the bubbles come up to the surface a film of foam will develop on the surface. Use a large metal spoon to skim away this foam. Let the shells cook like this for about an hour; skim the foam every few minutes. The foam comes from shells releasing impurities as their temperature increases.