As pointed out by @Jason, for most shared hosting environments, having a copy of php.ini file in your public_html directory works to override the system default settings. A great way to do this is by copying the hosting company's copy. Put this in a file, say copyini.php
<?php
system("cp /path/to/php/conf/file/php.ini /home/yourusername/public_html/php.ini");
?>
Get /path/to/php/conf/file/php.ini from the output of phpinfo(); in a file. Then in your ini file, make your amendments Delete all files created during this process (Apart from php.ini of course :-) )