WordPress 3.3.1 site is fequently hacked. How to secure site from Hacker?

I’m a wordpress custom theme developer and host wordpress site. From May 15, 2012, my wordpress sites are hacked. Now, I’ve resolved the hacked problems. Still the site not so secure. One site is hacked two times by different hacker. Five of wordpress 3.3.1 site is hacked by the same hacker. How to secure my site permanently form hacker?