Note: This is an old method, it may or may not work!
Method:
What you need:
-PSN Account
-Internet Connection
-PS Plus (If you don't have a PS Plus account you can sign up for the 14 day free trial here: http://bit.ly/1B0aO0P)
Steps:
1. Turn on your playstation and sign into your account then go to the PSN store.
2. Then after that go to the playstation plus tab , then go down to timed trials and click that.
3. Now there is going to be a list of games you can select. Choose any game you want and click download and then download in background.
4. Then keep pressing the "O" button until the screen prompts you with this, "Do you want to exit the Playstation store?" Say yes and it'll take you back to dash.
5. Go to network then download management and you should see your game downloading. Let it download to about 10 MB then press the triangle button and cancel the download.
6. Go back to the PSN store and go to search then type in the game you decided to download before. When you get to your game, just click download and now you should be downloading the full game for FREE.