Buy some of that cheap hollow yellow polyprop line they sell in hardware stores on a reel. Take 50' of it and run 25' of shock cord through the hollow center and knot off on both ends (those numbers are just an example - use whatever you need).
The yellow cover bunches up when not stretched and helps to tame the shock cord. It also greatly prolongs the shock cord life by keeping the sun and abrasion off. Since there is twice the length of cover, it stretches out when pulled.
Mark