Try some of these tips: http://blog.sucuri.net/2012/11/website-malware-removal-ftp-tips-tricks.html
Understand though that looking for backdoors is not a simple thing to do, the best recommendation I often give folks is reinstall core and all the themes and plugins and disable PHP execution in /Uploads
If you're not a developer, without engaging professionals to help, that's going to be one of the best solutions..
Tony