D.M.
My kids are older now, but we did it the "hard core" way - at about 22 mos., we did the "bottle ceremony" where we threw them away, and they got a special prize. It was hard for a couple of days, but they got used to it right away. You need to throw them all away for good, that way you have no choice but to use sippy cups. A child past age 2 should not have a bottle. We also had the binky ceremony a month later, that was harder, but they got over it, and they are fine today!