(Photo: Wikimedia Commons via CC Attribution/Share Alike)
Switch to Wi-Fi When Possible
In Berlin, I primarily relied on the free Wi-Fi offered at my hotels to get online, using my phone as a hotspot. (Simply go to "Settings," then "Wi-Fi," then turn it "On"—the iPhone will find all the networks available near you and show you which ones are "unlocked" for public use.) When I wasn't near free Wi-Fi, I turned the Wi-Fi setting off, as constantly searching for Wi-Fi can drain your phone's battery.
In London, many places (like coffee shops, pubs, and tourist attractions) offer free Wi-Fi under a network called "The Cloud." Once I registered (I used a fake email address to avoid spam), I could access free Wi-Fi in many places, thereby conserving my paid data plan.