Do: Bring an unlocked iPhone and a battery case
If you have, or can easily get your hands on, an older AT&T iPhone 4 or 4S, do two things: unlock it for free via AT&T's online unlock form, and plan for it to be your primary communication device during your trip. (If you have a Verizon iPhone 4S, the cellular company may also offer a free international SIM unlock for it, but you'll have to call.) Unfortunately, unless you had the common sense to purchase your iPhone 5 unlocked, AT&T won't unlock any smartphones (like the iPhone 5) that are under contract.
AT&T's form takes 5 to 7 days to process; if you need it unlocked right away, you can attempt to rush the process by calling AT&T's support hotline. I decided to do this at the last minute, and some very nice AT&T reps helped me out, but I'd generally recommend taking the time to fill out the form.
If multiple people are traveling, should they each bring their own unlocked phone? If you have the devices, I say yes. Our original plan was to bring just one unlocked iPhone 4, but we decided at the last minute to bring an unlocked 4S, as well. Though it was one more device to keep track of, overall it allowed us to have more flexibility throughout the trip.
When we got to Italy, we went to a TIM store and—in extremely poor Italian—managed to get two prepaid SIM cards with a gigabyte of data each. Minutes are pretty much useless unless you're communicating with someone in the country, but data is essential: It allowed us the freedom to wander cities at will without looking like tourists (by carrying around huge fold-out maps). 1GB of data each was good enough for 12 days in Italy, but your mileage may vary depending on where you're visiting, and for how long.
You may be able to find multiple carriers offering prepaid SIM cards; I was lucky enough to get a primer on Italian prepaid plans from our friend Federico Viticci, but if you don't have a native friend, the Internet can be incredibly helpful. (Lonely Planet often has good recommendations.)
Bring a battery case, too. You'll likely be far away from a power plug throughout the day, and the last thing you want is for your iPhone to die on you in an unfamiliar area with no way to return to civilization. (We used Mophie's Juice Pack line with great success.)
Don't: Post to social media throughout the day
We were very grateful to have a comfortable amount of data during our 12-day trip, but we quickly discovered that 1GB doesn't go so far if you plan on updating Twitter and Instagram with your latest photo and video snapshots. After a day or two, we limited ourselves to uploading and posting at the end of the day, once we were safely back on Wi-Fi. (If you want to make sure you're not wasting data, turn on Airplane mode, then switch Wi-Fi on from there.) Not only was this a good data-saving tactic, but it also kept us from busying ourselves with our phones rather than looking at Michelangelo sculptures. After all, vacations are a lot more fun if you experience them in the moment, rather than blogging the day away.
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