Horseshoes were originally made of iron, cause iron was supposed to give protection against witches, fairies and devils.
If you find a horseshoe you must follow a ritual to get the good luck. You must pick up the horseshoe with your right hand, spit on one of the ends of the horseshoe and make a wish. Then throw it over your left shoulder, and leave without looking back to see where it landed.
That a four-leaf clover means good luck is a common belief. It is not easy to explain why people believe so, but it may just be that since it's unusual people think it MUST mean something. Legends claim that the four-leaf clover was the only thing Eve was able to bring with her when she was expelled from paradise.