[widgit]

8 months-practiced with juice and water in a sippy cup.

9 1/2 months-switched from bottles to sippy cups over the course of 2 days.

Here are a couple of tips that worked for us:
1) Once you find a type of cup that works, use only that kind of cup
2) Try cups with a soft spout. Playtex makes one.
3) Try putting something yummy in the cup, but not in the bottle (i.e. water only in the bottle) We used a little bit of chocolate syrup with DD's milk in the cup for 1 1/2 days. That was just long enough to get her familiar with the cup.
4) stand your ground as previously posted.