Be confident. Walk up to her or when you pass her, keep it simple, like "Seen you a couple of times. I'm ABC from XYZ dept. It's a pleasure to meet you" ... then if there isn't any flow or continuity, just say "hope I bump into you again sometime, you have a great day, ta!" then walk away ..
It's a play by ear thingy and if you can pull it off .. the chat will be easier next round. If you ask about her dog, her cat, her entire family tree, her ex, her hobbies all in one go, you might as well just say goodbye to your chances.
Just an idea .. no hard and fast rules.
so i saw this girl in uni...