Well there is much ways to secure your dedicated server or isolate the accounts from each others , but you can never assure that it is 100% secure or isolated because they are already in the same system and you can't control a 0day bypass exploit for php or whatever services you are running , i think Virtualization or Cloud Based Solution would be more accurate for security.
but here is the most you should concern :
1. Kernel Update/Upgrade or patching it with GRsec ( not very useful if you don't know what you are doing )
2. Stop and Disable unused services.
3. Recompile Apache with EasyApache : use suPHP , suhosin , mod_security.
4. Disable Shell Access for all Accounts else who send you a Passport Scanned Copy.
5. Enable : open_basedir in Security Center ( WHM )
6. Use Strong Secure Passwords using the Generator for passwords.
7. Enable and make sure you always have backups ( preferred remotely )
The most important think is having Remote Backup as Server Security can never be assured 100% and due to disasters you can't also avoid natural disasters , disk failures or what so ever