Had an issue where I was not able to change the post_max_size and upload_max_filesize through the Apache2 configuration.
After googling around, I finally found what was wrong: apparently, aside of php_flag, there's also a php_value flag to set configuration variables. The difference?

Use php_flag only when setting boolean values like 0 | 1, off | on.

Use php_value to actually set values other than boolean (like the filesizes of post_max_size and upload_max_filesize).

Thus, a working example looks like:

<Directory /var/www/> <IfModule mod_php5.c> php_flag register_globals off php_value post_max_size 16M php_value upload_max_filesize 16M </IfModule><  ...