If you're seeing this error either suddenly (no specific task was done to cause the error) or frequently, try deactivating all plugins to rule-out a plugin-specific issue and try switching themes to rule-out a theme-specific issue.
Otherwise, here are three ways to increase PHP's memory allocation:
1. If you can edit or override the system php.ini file, increase the memory limit. For example, memory_limit = 128M
2. If you cannot edit or override the system php.ini file, add php_value memory_limit 128M to your .htaccess file.
3. If neither of these work, it's time to ask your hosting provider to temporarily increase PHP's memory allocation on your account. Keep in mind that most decent hosting providers allocate 64 MB to PHP under each account, and most decent hosting providers allow users to temporarily increase the memory allocation. If your hosting provider won't accommodate you, perhaps it's time to find a new hosting provider.
(in the above examples, the limit is set to 128MB)