1. If you are doing a package through Disney, your ticket will be full price, so the next item won't apply to you.
2. Buy a new ticket from a discount reseller.
Undercover Tourist is frequently recommended and if you sign up for the mousesavers.com newsletter they have a link to save you a little more. Choose the ticket that has the biggest dollar amount savings (doesn't matter which ticket it is). Note Undercover Tourist includes tax in their price and Disney does not when they display prices, so keep that in mind when deciding how much the discount is.
Then use the ticket to enter a major park (Epcot, Hollywood Studios, Magic Kingdom, or Animal Kingdom). After you have used the ticket to enter a park, then go to Guest Services and ask them to upgrade it to an annual pass. They should give you full gate price of your ticket and apply that towards the annual pass and you will pay the difference. If you upgrade before the ticket is used, you will only get the discounted price credited so make sure you use it first.
It doesn't save any money to upgrade on any particular day as long as the ticket has been used once. They will back-date the annual pass to the first date the ticket is used, so upgrading on the first day or last day won't matter. If you have used the ticket to go to Disney Quest or a water park, they will only let you upgrade to the Premium annual pass that includes those, so be aware of that.