I found it easiest to switch my son to the Nuby silicone sippy cup for milk, it was a very smooth transition from a bottle because of its silicon spout. I did the transition just after his first birthday, though I bet he would have taken it sooner.
After introducing it I always made sure that only milk was given in that type of sippy cup and that that was the only sippy cup given to him with milk. Kids crave structure and what better way than to help them learn expectations by making the contents of the sippy predictable?
By this time I had already established a water sippy cup with a hard spout that could be easily thrown without breaking open, the Nuk Gerber Sip & Smile Spill Free Cup. I tried the disposable sippy cups but the lid would fly off if he threw it on the ground so I went with these basic ones. It also helps to have water only cups so the milk and/or juice does not cause funky odors or tastes in the water.
As you can see, I not mentioned anythign about juice yet. That is because my son rarely has juice as it is a lot of unnecessary, albeit natural sugars and carbs. He might have a juice box if we are out somewhere or if at home I use The First Years disposable cups with the straw. Even though these will bust open if he throws them, he rarely does because he likes the taste of juice :-)