C.P.
I found that the most painless way was to make it less enjoyable.
If you're warming it, start by not warming it anymore. If it's milk, cut the amount in half, and start watering it down. 3/4 milk and 1/4 water the first night. The next night...half milk, half water. Etc., etc. When it's 100% water, start offering to her only in a sippy cup. Finally, when she's used to the sippy, start only allowing the cup to be in the kitchen or dining room, because that is where we eat and drink.
Best of luck!