As many people have mentioned, the one topic virtually all people love discussing more than any other is themselves. Their life, their opinions, their day to day activities. People will tend to like you more, or at least feel more positively towards you, if they feel as though you are interested in them. The easiest way to kick-start a conversation is simply to ask the other person a question, though sometimes that's easier said than done, especially in a social setting.
For parties and suchlike, where you're likely to try and make small talk with people you don't know or only know slightly, it's good to have a backup set of questions, like, "What's the worst job you ever had?" or something along those lines. Rather than just come out and say it like that, you can couch it as "some friends of mine and I were talking out the worst jobs we've ever had...I think the winner so far is (make something up)." You can then either ask them what theirs is, or they'll naturally suggest it.
You can also just do something really silly. Again, let's say you're at a party...try to figure out, from the current guests, the best people to be stranded on a desert island with. Or figure out who would play them in a movie. Or which room in the house is the best hiding place for the zombie invasion. Whatever. Unless the person with you is a total drip, you should be able to get some spark of conversation going.