Here’s the full solution in the cleanest possible way:

1. Create a file called php.ini in the root of your site (if you are using a hosted addon domain, this must be in the subdirectory of that site)
2. In php.ini, enter a line that says
   memory_limit = 64MB
3. In your site’s .htaccess (being a WordPress blog, I’m assuming there is one), enter the following line
   SetEnv PHPRC /<unix path to the directory where php.ini is>/
   (keep the slashes)
4. Edit wp-config.php and enter the following line
   define('WP_MEMORY_LIMIT', '64M');
5. Upload the new files to the server

Oh, and don’t tell your hosting provider you’ve done this…

租用的空间改php.ini 比较麻烦，按照第四点，改了wp-config.php，升级OK。