The airplane mode is actually the most efficient when it comes to energy. When you turn it on, most of the battery-consuming processes are switched off. That is why when you charge the device in that mode, it should charge up a little bit faster than in the usual mode. The ideal thing would be to turn your phone off when you are not using it however if you cannot do it, the aeroplane mode is almost just as good.
Usually the charger which you are given together with the device charges your phone the quickest. Charging by USB cable may be universal but they use less than 1 amper and the modern devices usually need 2 ampers or more. Simple math - the fewer ampres, the slower charging. That is why we recommend using the original charger. If you do not have one, there are special chargers which you can buy that have more than 2 ampers - remember to always check that parameter when you decide to purchase a new charger. In this case, Apple device are more thought through and it does not make much of a difference which charger you use with them.
Cheating? Well, maybe sometimes you should. One of the typical examples of how to trick your devices (and your laptop) is to use a cable with two ends and connect both of those ends to to USB portals. It is not better than using a normal charger but still quite a good solution.
On the market you can also find some devices - for example ChargeDr - which can be named charging accelerators. They will help your device charge faster when you connect them to a laptop. We also recommend outer banks of energey (now very popular power banks) which will save your device in cases of emergency.