As a hosting provider, you need to compartmentalize sites using cgroups(like cloudlinux).
As a siteowner,
- Keep website/plugins updated
- Use FTPS to upload files to your site
- HTTPS site for auth especially
- Change admin common login url if you can
- Dont use common/same passwords
- Hide showing error messages on page
- Disallow execute on upload folders
- Test for SQL/XSS injections