I have a bad back and after lots of Chi Sao, it aches and seizes up.
I make sure that my back is strong by lying face down on an excersize ball and lifting alternating hands and feet. Also, I keep my core nice and tight.
I have also found that keeping my hamstrings (which are connected to the glutes and lower back) nice a supple as well as stretching the hips and back very useful.
Just keep stretching and strengthening. If I miss a couple days of stretching, things start to hurt again.
Sincerely,
Kenton Sefcik
An ounce of action is worth a ton of theory. Friedrich Engels