Always use fresh eggs. If you can't see the difference between the "thick" white and the "thin" white, the yolks will probably break in the pan.
Always deliver the eggs to the pan with a custard cup or large spoon. Avoid cracking directly into the pan.
When using a non-stick skillet cook in no more than an inch of water. If you don't have a non-stick pan, poach in a deep saucepan containing at least 3 inches of water.
Always acidulate the poaching liquid with either vinegar or lemon juice (1 tsp per each cup of water). Bring liquid to a boil, add eggs, then remove from heat and cover. How long you ask? It depends on how many eggs. I like my yolks barely runny so I'll cook 4 eggs for 7 to 8 minutes depending on their size.
Since more eggs will absorb more heat from the water, they will take longer to cook, so for large batches always include an extra "test" egg. Always remove eggs with a slotted spoon.
Poached eggs can be refrigerated in ice water for up to 8 hours, then reheated in hot water. Do not re-boil.