Hi Annie
Not sure if you can prevent cording. I got mine after 1st chemo and followed all advice given, which was as you say, stretching and keeping the arm massaged. When I got home from chemo sessions I'd put either a hot water bottle or a wheat bag - one you warm up in microwave on the arm. My chemo was 3 years ago and you can still see the cording line.
Sorry not to be more helpful, good luck with everything.
Jude
x