There are 156 billboards in the game. Take the jump through them to smash and collect them. Billboards not yet collected have logos from various EA developers. After being smashed, it'll turn into a mini-leaderboard with the top friends Gamer Picture on it. This is how you will know if you've already collected a billboard.
Collect as many of these as you can as you play through the game. If you see one, find a way to smash it so you don't have to come back to it later. You can view all the locations of every billboard you've smashed by pressing and hitting until you get the billboards. Then, with the help of a guide, you'll be able to see which ones you've missed. Be sure to stick the landing; if you crash, you'll have to go back and try again.
This image has been resized. Click this bar to view the full image. The original image is sized 1082x685.
This map is from the Autolog website. Originally posted by our dear friends at ps3trophies.org.
You can also view an interactive guide HERE. It's less condensed, so it's easier to see where each one is. You can also check off the ones you've already found by clicking on them to help keep track of which ones you still need.