1. Install Firewall(For Managed VPS, Providers install this for you)
2. You need to know some SSH commands like 777 Folder Permission changer(Not Recommended but good for Server security /home)
3. Monitor Server Uses, RAM, Disk Space,etc and also check Quota,etc... If full then I think you will lose some clients.
4. Best Control Panel like cPanel/Plesk/DirectAdmin
Keep up to date your softwares, PHP, Apace,etc....