There are a number of safe ways to warm breast milk or formula, with safe being the key. This said, microwave use for warming baby bottles is strictly not recommended. Your microwave creates hot pockets in foods that can remain even after shaking a liquid, such as formula. Think of taking a sip of scalding hot coffee, or a bite of food that should have been cooled a little longer. It can be very painful. Why take the chance on a painful burn, when there are other ways to go? Besides, did you know that heating breast milk can destroy nutrients?
First, it is perfectly fine to give both breast milk and baby formula at body temperature or cooler, so the use of any heating may be a moot point, though baby may prefer a cooler bottle in warmer temps, and a slightly warmer bottle when there's a nip in the air. If your formula fed or breast milk fed baby prefers a little warmth, try running a bottle under warmer tap water with the cap on, checking temperature frequently. Or place a bottle in a pot or pan of heated water that has been removed from the flame. Shake to mix, check bottle and remove once desired temperature has been reached.
When away from a kitchen, a baby bottle warmer set is an ideal way to quickly get a bottle ready for a late night nursery feeding. When traveling, having a portable baby bottle warmer that simply plugs into a cigarette lighter can be a lifesaver. They're a little more time consuming, so plan ahead, but great when on the go. Most are adjustable for various sizes, and after bottles become things of the past they serve as great travel mug warmers! There is a great selection of warmers on the market, with adjustable opening for various bottles, ones for bottles and baby food jars and some that ate capable of warming expressed milk bags without melting the plastic.
No matter which way your baby's beverage of choice is warmed, it should always be tested on the inside of a wrist for temperature appropriateness prior to giving it to baby. It should not feel hot, just warm.