Heatshrink sleeving usually works, depending on how large the DC jack is. You need to use adhesive lined, with at least a 3:1 shrink ratio (I think you can get 4:1). Use the smallest diameter that you can persuade over the DC jack - you can achieve a brief further expansion by stretching sleeving over the jaws of some round nose pliers.
Shrink the sleeving over both the strain relief and the cable outer - if the bit over the cable doesn't shrink enough, you can squeeze the sleeving flat at each side while still hot. The result will provide both mechanical support for the weakened cable and some protection against further pulling.