A tactic that I love to use when meeting new people, especially for those that I don't have a "reason" to introduce myself to such as other students on their way to class or someone standing in line next to me, is to ask about or compliment something about them. For instance, I might ask a gentleman "Wow, that briefcase is a beauty! May I ask where you got it?" "Excuse me, Miss, I just wanted to say that I love the look of your jacket/purse/necklace. Would you mind telling me where you got it? I'm gift shopping for my girlfriend/wife/sister, and I think they would love one like that."
This tactic takes away from the awkwardness or surprise of being approached by a stranger and guides the conversation towards something they're already knowledgeable about and comfortable with. From there, you can launch into deeper conversation about where they're from, where they work, what they study, what their dreams and goals are, etc.