A seed planted at its correct depth, with the ideal soil temperature, amount of light and the right moisture level still may not sprout if it doesn't break dormancy. In nature, a seed breaks dormancy in one of several ways. Some seeds pass through the digestive tract of an animal while others are triggered by warmer temperatures following cold winter weather, heavy spring rains or ample sunlight. To encourage seeds to germinate, humans must sometimes imitate these natural processes.
In some cases, seeds need stratification, or chilling, before they can germinate. This chilling period imitates cold winter temperatures. Without this cold requirement, certain seeds might sprout in the fall once they reach maturity only for the tender young plant to be killed by frost or snow. Immitate this natural cold period by placing seeds in a bag with moist sand, peat moss or paper towels and refrigerating the seed at around 40 degrees Fahrenheit for several weeks to months; the required minimum time varies between plant species. In temperate areas where winter temperatures are sufficiently cold, seeds are simply sown outdoors in fall, although this doesn't give you as much control over the seed's environment as artificial stratification.
Sometimes a seed's dormancy is caused by a heavy seed coat rather than an internal trigger. In this case, the coat must be physically broken or disturbed in order for germination to occur. In nature, a seed coat breaks down by passing through the digestive tract of a bird or other animal, weathering, fire, soaking in water or through other means. You can imitate these techniques that include by boiling seeds or soaking them in water, scratching the seed coat with a file or sandpaper, cracking the seed coat with a hammer or soaking the seed in vinegar or a stronger acid.
Some seeds need light to break dormancy and stimulate germination. Seeds that require light must be sown on top of the soil or medium and, if they are covered with any medium, it should be no more than a fine dusting. Removing the outer tissue of any seed that requires light to germinate permits the embryo to germinate even without light. Some seeds need a specific type of exposure to light, with long or short days, while others only need brief exposure or multiple periods of light to break dormancy.
Some seeds need after-ripening -- a long drying period when the seed needs to reduce its moisture content to a certain amount -- to break dormancy. In nature, seeds like this can remain dormant for several years under dry conditions until certain chemical changes occur within the seed. Humans can artificially speed up this process by maintaining a temperature of about 100 F around the seeds for five to seven days.