Drush memory exhaustion
Drush includes a sample php.ini file (there's an online copy here) with instructions on how to override your the php.ini file that Apache uses.

Just copy the contents of the example file into one of the three locations listed in the sample file and save the new file as php.ini. I would recommend using either option except for your Drush installation as this will require you to re-create a php.ini every time you update Drush.

Running drush status shows you which PHP configuration file it is using in case you're unsure if your changes took effect.