Address Book is one of Apple's apps that might seem too basic at first glance to many a user. While this under-appreciation could largely be due to its unassuming name, Address Book offers a surprising amount of useful features, especially in the contact management department. Of course, there are the typical Apple-esque Smart Groups which you can use to automatically and dynamically round up contacts based on any criteria you want (family last names, employers or even notes you add yourself), but check out the Card menu in Address Book and note two options: Merge Selected Cards and (my personal favorite since I play with way too much synching software in the name of TUAW and Download Squad) Look for Duplicate Entries. Those are two great tools for ensuring your contacts only take up one entry each in Address Book (if you only want
one entry for each, that is), and I highly recommend using Look for Duplicate Entries if things get out of hand, as it's saved me countless hours after botched synching sessions and wayward synching software had their way with my contacts.
Importing new contacts cards (usually .VCF files) is nothing very special, but Address Book is pretty smart when you import a card for someone already in you contacts list. Typically, Address Book will catch the potential duplication and offer you four options for managing both the old and new cards. A window will be presented with four buttons along the bottom: Keep Old, Keep New, Keep Both and Update, allowing you to ensure that you don't blow away old data you want to keep for the contact while importing anything new you might need.
Combine some of these tricks and tools with the Mail + Address Book tips
Merlin Mann wrote about back in April, and you have some pretty powerful tools for managing your contacts and organizing the correspondence you have with them.