This is quick and simple. Don't.
How many times have I seen a company doing nightly backups of Citrix XenApp servers to tape? What for? In case something happens and we need to restore it. Unfortunately, I have never seen a Citrix XenApp server successfully restored from a tape backup.
Image based backups taken off shutdown servers work very well but traditional backups done with tape from running Citrix servers fail every time - (in my experience.)
You should not have any data on your your Citrix XenApp servers (Presentation Servers) so when they die you just rebuild them. Normally this would be done with some sort of imaging software like: Ghost, Altiris, VMware snapshots, ADS etc. (The free Windows ADS works quite well so I like that.) Applications and patches can deployed using the Citrix Installation Manager from the Citrix Management Console.
You should have an automated build process for your Citrix XenApp servers that allows a quick restore in a disaster.
Just a small reminder that the Citrix server software has to be installed not cloned as the act of installing the Citrix server software is what registers the server with the Citrix Data store.
Backing up your Citrix servers is a waste of tape.
Think of the application servers as expendable servers that when they have a problem they are blown away and rebuilt. Then put your effort into a robust and repeatable rebuild procedure.
Also note that Citrix server degrade over time. Too many users doing weird things. If it has been running for 2 years and you are having a problem with it just blow it away and rebuild it. You should be able to rebuild a dozen Citrix servers in an hour. On a site I am working on we can rebuild the 160 servers in 4 hours.
DATA STORE
Hopefully this will be on a SQL Server or Oracle database and should be backed up using an appropriate backup program with the correct database agent installed. Losing your Citrix DataStore could be a real problem, especially if you have lots of published applications.
CITRIX LICENSE SERVER
Citrix servers can survive 30 days without seeing its license server so this is not critical and if there is a disaster you can rebuild this at your leisure so long as it has the same name as the original because the Citrix license file is encoded for a particular server name.
Keep a backup of your Citrix license file on your file server.
TERMINAL SERVICES LICENSING SERVER
If you don't have a Terminal Services Licensing server you won't be able to start Citrix or terminal Services sessions. You can install the license server and activate it easily then take up to 90 days to get your licenses on to it. You will get temporary licenses issued to you workstations for the first 90 days. With Win 2000 you had to put the TS license on a Domain Controller but with Win 2k3 you can put it on any member server. However it will only auto-discover the TS license server if it is on a Domain Controller. You can use Active Directory to direct the servers in a particular OU to a particular TS license server. Backup is not required as you can install the TS licensing service in less time than you could get a Windows server running form a backup.
WEB INTERFACE SERVERS
Keep images of them so you can restore from an image. These servers are very good candidates for XenServer or VMware. Although reinstall is fairly quick and there is not much configuration on them. Just document the config.