DS watches and copies me. And big children at nursery (I have to introduce everything so early otherwise I miss it!).

What I do is give him his cup (plastic with a dinosaur on) and his sippy cup. I pour a little bit of water from the sippy into the big-boy cup. DS then drinks from his cup. DS then pours himself another drink. Yes, it's a bit messy but it's only the high-chair tray, that's wipe-clean.

Also, right from the start he has drinks in an adult glass sometimes (usually because I forgot his sippy cup, bad mother rather than proactive mother). I hold it and he holds it and he drinks - but I keep him from slopping all the water all over him. Doesn't work now, I'm not allowed to touch the glass, but he has learnt not to pour water down himself.

He doesn't get any sugars on his teeth from juice because we don't drink juice!