I never had any luck getting my daughter to "pose" at that age - my best pics are when she was laughing about something and sometimes I would try to get her to laugh anyway I could and snap away. Making funny noises or faces was a good way to get her attention, focus on me for a second, and laugh or smile. It also helps to get down on their level, and have your shutter speed set as fast as possible! Also, often the best pics are when they are doing their own thing and having fun, like playing outside or in the tub. I have always tended to take a lot more "action shots" and they often end up being the best ones.
Now DD is 4 and rarely cooperates for pics - I think it is actually harder now! She never wants to hold still, or look at the camera and she fidgets all the time. She also does this fake smile/grimace thing so I still need to try to get her laugh or really smile about something so it looks genuine!