Auto-complete can save a lot of time in addressing emails, but sending a tossed-off "Can't wait for this day to end" to your boss Rick instead of your friend Rick ... well, that's trouble. Rob Griffiths at Macworld offers his simple solution for avoiding this in OS X's Mail, but it's valid for nearly any email client:
In my case, I created two new groups in Address Book (File -> New Group, or click the plus sign in the lower left corner). I named one da boss (because, well, he is) and the other wrx (which is the brand of car my friend Jason owns) ... I then dragged Jason Snell's contact record into da boss group, and my friend Jason's card into the wrx group.
Tips on other methods to keep contacts separate are welcome in the comments.