Easter Lilies are common perennial plants that grow each spring from a bulb. They are know to be able to tolerate weather extremes that many plants can't handle. It is necessary though to protect the plant from cold winter winds and sunlight, so often times in the fall it is a good idea to make sure that the ground is well drained and covered with a mulch of some kind. Mulches will also help prevent bulb exposure from heaving of the soil during the winter and spring.
It is important not to plant the lily too early, wait and plant it after the chance of frost has pasted in the spring. Choose a sunny garden spot and remove the plant from the pot by turning it upside down, holding the plant top and tapping on the side of the pot. Then plant the bulb a little deeper than it was in the pot, opening the roots and stirring some soil in around them. Water it well and add a half teaspoon per gallon of 10-10-10 fertilizer to help initiate root growth. The old top will die eventually and then new shoots will appear. Later on in in the summer they will flower, as long as trees, and other things don't shade the lily.
It is best to remove the flowers as soon as they start to wither. Also trim any brown leaf ends. If the plant is in a pot, it is okay to store it in a sunny window or keep it in the basement window until the danger of frost is gone. If the leaves start to yellow and droop, it is just going into a rest period. One other important point to note is that it is essential to keep the lily on the dry side to help prevent rots.
http://www.ag.uiuc.edu/~robsond/solutions/horticulture/docs/eastlily.html
http://aggie-horticulture.tamu.edu/plantanswers/publications/lily/lily.html