Download Line2, an app that adds a working line to your old iPhone. For $10 a month, you get unlimited VoIP calling and text-messaging over a Wi-Fi connection. Sure, you can run Line2 on your iPhone 4, but it's handy to have a number for people you don't want bothering you on your regular line. If you're already a Skype user, get a dedicated SkypeIn phone number ($60/ year; skype.com) instead. The extra line lets you save your active iPhone's battery and minutes. And if you plug your two numbers into Google Voice (free; google.com/voice), when people call you at home, both iPhones will ring, so you can manage your minutes by choosing which one to answer.