Enable airplane mode
When you activate airplane mode, your phone automatically disables both the wireless and cellular connections inside the device, which significantly reduces how much power it consumes. That means that since it doesn't have to focus on keeping the phone itself running at full power, charging is more efficient. Seriously, it works.
Take your phone case off
iPhones are notoriously heat-sensitive, and it turns out that they have to work extra hard when they get too hot. And since many standard cases can cause them to run a little hotter than they otherwise would, it's wise to let them charge naked.