Here's the best method I've found, and it works well for iPhone and Android smartphones, as well as tablets such as the iPad.
#1: Use a good quality, powerful charger
One of the fastest chargers I own is an iPad charger, and I use this to charge up pretty much all my devices as it cuts down the amount of time my devices are hooked up to a cord.
And the difference is quite staggering. For example, my iPhone 6S Plus takes over three hours to charge up from flat to full using the supplied iPhone charger, but that time is slashed to just over two hours using an iPad charger.
Another charger I've had good success with is Amazon's 9W PowerFast Kindle charger.
And no, I don't find that fast-charging devices has a negative effect on battery life (the charging circuits controlling modern batteries are very good). I've been doing this for years and not had any problems.
But if you're still worried, just use the charger that came with your device.
#2: Put the device into airplane mode
By doing this, the device is not burning through the battery as you're trying to charge it up. On average, I find that devices charge about twice as fast when they are in airplane mode as they do when in normal mode.
Essential Android and iPhone accessories
#3: Remove any cases
This is especially true if it's a thick rubber cases, because batteries charge slower the warmer they get. It's also a good idea to put the device in a cool spot while it's charging.
#4: Leave the device alone while it's charging!
Resist the temptation to check how it's charging every few minutes, because each time you activate the display or use Wi-Fi and such, you're consuming precious power!
#5: Carry a small portable battery pack
A small pack such as the Anker PowerCore mini carries enough charge to fully charge an iPhone 6S in about 2 hours, and yet is small enough to slip into a pocket (remember that you'll need a cable too!).
Five iPhone battery-saving tips that really work (and several that are useless)