If the cable is made of latex rubber, it can be softened by brushing with methyl salicylate (oil of wintergreen). This also delays the decomposition of the rubber. If it is made of a synthetic polymer like PVC, this will not have any benefit.
The plasticity of synthetic rubbers can sometimes be improved by boiling them in water for a few minutes. This redistributes the plasticizers. Scrubbing can also improve the texture, as the above poster noted: by taking off the cracked outer surface a new, smoother surface is revealed.