Kerry D. Wong has a simple solution for adding strain relief to existing cables: heat shrink tubing! If they’ve started to give you can use this technique to likely save them, and if you suspect a cable will become weak over time you can do this early on to save it from itself!
Not all cables have proper strain relief built to the connector ends. This is particularity true for some of the iPhone 6 USB to Lightning™ cables. Take this Anker branded iPhone cable for example, after several months’ normal use the cable towards the USB end started to show signs of excessive strain (see image below). The internal wiring would soon be damaged if the strain on this area continues.