There is a post on the CentOS How To's about how to secure SSH on a system. Section five talks about using a non-standard port:
- Use a Non-Standard Port
By default, ssh listens for incoming connections on port 22. For a hacker to determine ssh is running on your machine, he'll most likely scan port 22 to determine this. An effective method is to run ssh on a non-standard port. Any unused port will do, although one above 1024 is preferable. Many people choose 2222 as an alternative port (as it's easy to remember), just as 8080 is often known as the alternative HTTP port. For this very reason, it's probably not the best choice, as any hacker scanning port 22 will likely also be scanning port 2222 just for good measure. It's better to pick some random high port that's not used for any known services. To make the change, add a line like this to your /etc/ssh/sshd_config file:
# Run ssh on a non-standard port:
Port 2345 #Change me
Then when using SSH to connect just specify the -p switch if your doing it from another Linux server. Or if your using something like PuTTY change the port there.