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>
  </Directory>