We're using Playtex drop-in bottles, and they say not to microwave. I don't know much about bottle warmers because our daughter would drink room-temperature milk until just recently (she's 1 y.o. now and we're trying to get her to sippy cups rather than bottles).
The only two ways I've tried are (with 1 being my preference):
- Heating up water in a coffee mug, then putting the milk filled bottle in the coffee mug full of hot water. (This keeps me from heating up the milk too hot)
- Heating up the milk in a microwave safe container, then pouring that into the bottle liner. I've gotten it too hot this way a few times and had to cool it off.