Hi Hulio, honestly I have no children and haven't actually experienced what you're describing. I do however watch a lot of TV, and one of the reality shows is about a professional very experienced nanny who comes to houses, stays for a week and changes the life of the family, concentrating on the kids' beahviour of course. In one of the shows, all 3 kids in the family - who ages range between 2-6 (!!) years all still use a sippy cup. i recall the nanny opening like 5 drawers filled with thousands of sippy cups :) Anyway - she immediately decided no more sippy cups and the way to do it really is simply cold turkey... just take it away. The kids naturally cry and cry and get mad, but you tell them they are grown ups now, and no more sippy cup and after a day or two they drink out of real grown-up glasses :)
Again - this is from a TV show :) so if i were you, i would consult some more.
Good luck!