Hi Brian.
I'm a DVC member with an annual pass. I've always chosen to buy and renew my AP right at the parks. You can do this at guest services and it only takes a minute. I feel like if I'm watching the transaction I can make sure that I get I get the DVC discount I'm entitled to.
Which bring me to ... HAVE YOUR DVC MEMBER CARD WITH YOU WHEN YOU BUY YOUR ANNUAL PASS. This is how you can prove that you are a DVC member and qualify for the very substantial DVC discount on annual passes. Currently this discount is over $100.
If you wanted, you could also purchase your annual pass through the DVC office at dvcmember.com. In this case you would be buying a voucher that is mailed to you. To activate the voucher, you'd still have to bring it to guest relations at a park, so you're not really saving any extra steps.
While you're getting your annual pass, you may also want to consider purchasing a Tables in Wonderland discount card. This card is only available for Florida Residents and annual pass holders. You can read more about this
HERE and
HERE.