It's normal for them to stretch a little when new, but it should only be needed a couple of times. After that it should just need adjusted as the clutch wears.
Check the ends of the cables for fraying where they connect to the barrels, that's where they break.
And be sure to keep enough free play in the adjustment. As the engine torques it pulls on the cable, so if you don't have enough slack it constantly has the cable under tension. I adjust mine so it has 2 or 3 quarters worth of free play at the lever gap. Basically, turn the adjuster out so there is no gap, pull the lever and insert two stacked quarters in the gap, then turn the adjuster in until the quarters just fall out.