How can I configure my User/Group/Permission setup for shared webhosting on linux to prevent permission between webserver and user accounts?

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http://serverfault.com – I have one Linux LAMP webserver (Ubuntu) with some websites on it. They all run within the same apache2 instance and are accessible by different vhosts. Most of them are run by myself, but I also have some friends websites hosted there. I want and use to have separate user accounts for different websites to manage files, cronjobs etc. The webserver runs as www-data. Now most of the files/directories don't require write permissions by the website, but some do. I am okay with separately setting those to writable with the user I use to manage the files. In fact i prefer it that way. Howeve (HowTos)