How to create a glance image from a running VM?

Hi all,
Here's a question on Openstack usage: How to update my glance image with new version of software?
In Openstack, I can easily create a instance from a image, this image usually contains most softwares I will used. In this running instance, I have to store all my data outside of it because all data will lost when I shut it down.
However, if I updated the softwares, I'd like to update the image in glance. I know it is impossible to modify a existing one, I just want to upload a new one. When the softwares are updated in VM, how could I create a glance image from that VM?
I tried to invoke 'nova image-create', but I found it just dump the VM into a Volume and boot new VMs from that volumes which I absolutely don't want to see.