When i want to make someone smile, usually i give that person a compliment. if you're going for casual, you could say something like 'I really love that shirt, where did you get it!?' or if you're going for flirtatious, try something like 'I love your eyes.' Most of the time, it works!
p.s.-if compliment form doesn't work, trying doing or saying something humorous, a good laugh gets people smiling