Disney has an upsale at the ticket counter, called "Park Hopper"
It's usually about $25 additional dollars to your ticket per day.
Personally, unless you are an EXTREMELY experienced Disney traveler, and you know the parks inside and out, I would skip this option. When you are at the parks, you are up against the clock. You pay nearly $100 per ticket per day, and you only have 9 to 12 hours to get in as much as possible before the day is over.
Attempting to "park hop", takes about 3 hours out of your day. It will take you about an hour to exit the park you are in (Epcot for instance), an hour to travel to the other park (via monorail or driving), and about an hour to get through the entrance lines and into the new park. 3 hours might be a bit exageratting, but this does take quite a bit of time to exit one park, and enter another. Compound this if you have kids, or if you are unsure where to go, or how to get there.
Personally, I suggest skipping this option until you get EXTREMELY comfortable with the parks, monorail, bus system, and parking system, but by then you probably should just have season tickets.
Pick one park for the day, and stay in that one park all day. Get to the park, make a bee-line for the ride you want to ride, pick up the fastpass (you can only get one at a time) and continue touring the park.