First, try installing the Memory Bump plugin.
If that doesn't work, here are four more ways to increase PHP's memory allocation:
1. If you have access to your PHP.ini file, change the line in PHP.ini
If your line shows 32M try 64M:
memory_limit = 64M ; Maximum amount of memory a script may consume (64MB)
2. If you don't have access to PHP.ini try adding this to an .htaccess file:
php_value memory_limit 64M
3. Try adding this line to your wp-config.php file:
Increasing memory allocated to PHP
4. Talk to your host.