One theory of this is that during the Hundred Years War between France and England, archers would cross their fingers before pulling the bow string in order to grant them good luck. Before that, it was also a secret sign between members of Christianity (when it was illegal).
Never take the third light from a match, it's bad luck.
In World War I, snipers sometimes operated at night. Their technique involved waiting for someone to strike a match to light a cigarette; on spotting the light they'd train a scope on them. On the second light, the sniper would focus his shot, zooming in ready for the kill, on the third light, he would fire, killing the person who lit the match.
Walking underneath a ladder is widely held to be bad luck.
'Walking under a ladder' came about in England a few centuries ago when people drank more ale at lunch than ate food. Drunken sign painters were likely to dump a bucket of paint if you walked by or under their ladder and handymen would drop tools.
This was part of World War II propaganda. The British had invented radar and we began bombing the Germans at night. The Germans, confused as to how we bombed them at night, started researching how we were doing it. The British government began to spread the rumor that it was the carrots in our ration packs that helped us see in the dark (Note: "in order" is rarely needed) to put the Germans off our trail. They figured it out eventually, but by that time, the rumor had already stuck.
Spilling salt is bad luck because in Roman times, salt was so valuable that soldiers were paid in salt rather than money. Spilling it was equal to burning money. I'm not sure where throwing a pinch of salt over your left shoulder negates the action of spilling, but I can tell you that it is where the word 'salary' came from.