[Arm-dev] making a compressed image

I have worked out the dd command to copy my work in progress 4Gb image:

dd if=/dev/sdb of=cubietruck.img bs=512 count=6268927

and restore it with:

dd if=/home/rgm/arm/Centos/cubietruck.img of=/dev/sdb bs=4M; sync

But I would like to compress this, making a .xz file that is expanded 
with xzcat.

I have not figured this out, so am asking for a bit of help (I have had 
to increase the  root partition so backing up is a larger file now).

thanks