To secure your NSX deployment, it is important to back up the vCenter database and take snapshots of the VMs.
Refer to the vCenter documentation for your vCenter version for vCenter backup and restore procedures and best practices.
For vCenter Backups, see the following:
The vSphere Installation and Setup documentation for your version of vSphere
For VM snapshots, see http://kb.vmware.com/kb/1015180.