How to disallow files execution in upload directories?

[postcd]

Hello,

one member adviced me that "You need especially to disallow execution of any files in your upload directories (avatars, profile pics, etc.)"
because my site got hacked, some script files modiffied by malware redirect.

So i want to ask how i can achieve this? I know how to chmod directory, but in the upload directories are added new files as time goes, and how to automatically change their permissions, or how tis meant?

attachments/
customavatars/
customgroupicons/
customprofilepics/
signaturepics/

What is proper above folders permission? 777 not? thanks for advice