Try one (or more) of the below methods to increase the memory allicated by PHP. First try increasing the limit to 64MB, and if that fails use 96M.
You can define the WP_MEMORY_LIMIT constant in wp-config.php:
Increase PHP Memory to 64MB
define('WP_MEMORY_LIMIT', '64M');
Increase PHP Memory to 96MB
define('WP_MEMORY_LIMIT', '96M');
You can also change the setting in your .htaccess file:
Increase PHP Memory to 64MB
php_value memory_limit 64M
Increase PHP Memory to 96MB
php_value memory_limit 96M
You can add a line to wp-config.php telling PHP to increase the memory limit:
Increase PHP Memory to 64MB
ini_set('memory_limit','64M');
Increase PHP Memory to 96MB
ini_set('memory_limit','96M');
This is not the most preferred method, and you should try the other methods first.