Question cannot easily be narrowed so has been removed - please delete.
There are either too many possible answers, or good answers would be too long for this format. Please add details to narrow the answer set or to isolate an issue that can be answered in a few paragraphs.If this question can be reworded to fit the rules in the help center, please edit the question.
https://www.digitalocean.com/community/articles/how-to-install-denyhosts-on-centos-6 https://www.digitalocean.com/community/articles/how-to-setup-a-basic-ip-tables-configuration-on-centos-6 https://www.digitalocean.com/community/articles/how-to-protect-ssh-with-fail2ban-on-centos-6
Any other tips or advice would be greatly appreciated, as would links to guides or how-tos.
Get registered on some good linux forums and search around for 'hardening' your partifular distro and deployed applications.
Feel like a bit of a phoney posting this as all credit must go to the excellent Digital Ocean