Use data detectors to create/update contacts
Mac OS X Leopard and Snow Leopard offer Data Detectors in Mail. This means that the Mail app is scanning the message you're viewing for common things like phone numbers, addresses, dates, email address etc. You can use these Data Detectors to update your Address Book. You can use them to either update an existing contact or create a new one.
Use notes to keep other info about the contact
I use the notes field at the bottom of the contact record in Address Book all the time. It's great for additional information about the contact that I don't want to necessarily create a field for. For example, one of the uses I use it for is to put in the hours of operation for a business or store. Since notes also sync to your iPhone/iPod this information will be displayed when you bring up this contact on your portable device too.
Customize your Address Book Template
The default Address Book template was missing fields that I use regularly and had ones in it that I never use. So I was tired of adding the ones I needed for each new contact. Instead I simply customized the Address Book template to what I needed for each new Contact. Choose Card->Add Field->Edit Template from the Menu bar.
Setup a "My" card
I know it sounds silly to create a contact for yourself, but this is actually a useful feature that gets used throughout Mac OS X. If you've ever wanted to have your default email address, home address, phone number entered when you're filling out a form on a website in Safari, then setup a contact in Address Book with all of your default info. Next choose "Make this My Card" from the Card menu. Now anytime the OS needs your info it will grab it from here first.
I use groups for lots of things (it was great having a "Holiday Card" group). The most common thing I setup groups for is to quickly email a number of people that share a common interest. For example, "LOST fans". This way when I find a cool thing about LOST (the TV show) I can simply key in "LOST fans" in the BCC field of Mail and email it to those folks quickly.
Bonus Tip: Which email address does a group use? If someone in your group has more than one email address you can dictate which email address is used when you Mail to this group by choosing Edit Distribution List from the Edit menu and then when you select the group you'll see the contacts with multiple email address and you can then choose which one to send to when you send to this group. Great when you want it to go to their home email address instead of their work one.
Add contact photos
I'm a visual guy and I depend on photos wherever I can get them. So I go out of my way to add photos to each of my contacts. This helps me when they call their picture shows up on my iPhone and when I get an email from them their picture shows up above the message. Now I know I told you about this when I wrote my 10 Mail Tips post. So that you don't feel cheated (out of your free tips 🙂 ), here's an additional one for this tip. Where do you get the photos? If they are your friends on Facebook you can grab them from there. Most Facebook users have profile pictures. You can actually drag their picture directly from Safari into the Contact photo area of the contact you're editing.
Put a comma in for a pause
I used to struggle with where to put in someone's phone extension. The problem was that if I simply put it on the end of the phone number, when I dialed it from my iPhone it would just dial it too. I would then have to physically look at it again to dial it when prompted to do so by the phone system at their office. Now I put the extension in after the phone number but I put a few commas in-between. Each comma will cause a 2 second pause before the iPhone dials the next number. This gives the system you;re calling on the other end time to answer and wait for the extension.
Use Smart Groups
Groups are cool! They're even cooler when they update themselves. You can create a Smart Group that will continuously update based on the criteria you set. So if you want a group of contacts that all work for the same company, just create a Smart Group to do that. Whenever you add a new contact with that company name, they'll automatically be added to the group.
Get the cool plug-ins
Address book is great but I found a couple of plug-ins that make it even better. I love having the US Post Office corrected addresses for my friends, but I don't like looking this info up manually. I don't have to. There is a cool PostCheck plug-in for address book that will automatically correct the selected address and add the Zip+4 for you. It's $10, but for me it was very worth it.
Back it up!
If I lost all my contacts I would be dead! I have 1,174 and I couldn't imagine having to start over. Yet people lose all of their info every day due to hard drive crashes and other mishaps and I don't get why people don't backup? Luckily if you're on Leopard or Snow Leopard you've got this great thing called Time Machine. Also if you're using MobileMe and syncing to it your Address Book is being sync'd to the cloud. I still want more protection! So I use the Export Address Book Archive feature from the File menu. This will create a single file of all of your contacts that you can store in a safe place (hopefully offsite). If your contacts ever got screwed up or lost you could restore them from this file. Again Time Machine will do this too, I just like having another backup.