You have to book sold out / overbooked flights. Otherwise no upgrades.

Economy is rarely oversold on most routes - too many seats to fill. You have a better chance when booking PE as it is a small cabin and some companies have no Business Class policies so PE can fill with late walk-ups/flexible tickets.

But ultimately you should always expect to fly in the cabin you paid for. Check out the Premium Fares thread for ideas about how/where to book Business Class at a reasonable price in the first place...