The most reliable way of getting upgraded for free is to have elite status on the airline you're flying. Most airlines reward their most frequent flyers with complimentary upgrades on domestic routes.
Is it easy to upgrade from economy to business class?
Realistically, it's a gamble and getting upgraded to business class at the airport is pretty rare. If luxury and extra legroom is really important to you, book it in advance. However, if you're perfectly willing to fly economy class, and willing to roll the dice, a last-minute upgrade can be the cheapest option.
Why do airlines sometimes upgrade for free?
In rare circumstances, a flight may be oversold in the economy cabin, but there could be some space in first or business class. In that case, some very lucky passengers might be bumped up to first or business class for free in what's known as an operational upgrade.