Light the haystack on fire. Needle, being made of metal, will not be easily harmed by the fire. Once the hay is burned to crisp, you will have no problem finding the needle. While that may seem a bit extreme, you have to remember that the task of finding a needle in a haystack calls for extreme measures. Since hay is dry, you will not need to use any flammable liquid such as gasoline to prepare it for combustion. Just light up a matchstick and throw it in. Make sure you set the haystack on fire in an open area with no dry grass or branches nearby, or you would find yourself biting off more than you can chew.
Another effective method of finding a needle in a haystack is by using a magnet. Metal is attracted strongly to magnet while hay is not and that is something that you will be using to your advantage. The only issue, however, that you could be facing while dealing with a haystack of considerable size is that an average-sized magnet will not prove to be very effective. You can, however, overcome this problem by somehow getting your hands on a very strong magnet. Spread out the haystack and then move the magnet across it. With a bit of patience, you will successfully find the needle.
Use the difference in the density of a needle and hay to your advantage. A needle sinks in water despite its small size and extremely light weight. Hay, on the other hand, floats because of having a lesser density than water. Therefore, fill out a large tub with water. Pick up some hay from the haystack and throw it in the tub. Put your hand inside the tub and move it across the bottom to feel for the needle. Continue putting the hay in the tub and feeling the bottom.