The Drush enjoyment doesn't stop with single site installations. To ensure typo-free commands in multi-site environments, I have set up additional aliases in .bashrc.
alias drush1='/home/account/drush/drush -l http://example1.com/' alias drush2='/home/account/drush/drush -l http://example2.com/'
This allows for a command like drush1 status or drush1 update to quickly work on the first web site, while drush2 update automagically knows to only update modules on the second web site. Add aliases as desired!