Sony has patented and idea that is so obvious you wonder why no one else has thought of it. The basic premise is rather than download a demo of a game, you download the complete game. As you play the game features get removed, slowly crippling the game back to demo status.
Taking the recent Sonic & Sega racing game an as an example, you would download and play the game with access to all tracks and characters, but as you continued to race, tracks would be locked off, multiplayer versions would vanish and characters would wave adios until you were left with one track and one character. Pay for the unlock key and your demo becomes the full game again.
Another example is illustrated in the picture below. You can make you own jokes.