Lookout for things like making sure your server has enough bandwidth, memory and processing power.
You can check things bandwidth utilization and if your at 95% you obviously need to upgrade the speed of your servers.
If you need more ram / space buy more!
You will need to make sure you keep the server secure by installing the latest patches etc, oh also keeping backups of the clients data would be a good point to make in case the server messes up
would be a good place to look round