Airline upgrade offers vary among airlines. If you're looking at a specific one, the best advice will be found in that airline's FlyerTalk forum.

In general -- and this is highly general -- (may not apply to each airline), there are four ways to upgrade besides purchasing a ticket in the upper class:

1) Use of miles. Airlines allot certain seats for the use of these. Your place on the list will be below the number expected for purchase and may be below those of higher status.

2) By status. Airlines may offer complimentary, space-available or confirmable-in-advance upgrades to psgrs. of highest status with the airline. These might not clear far in advance of the flight and won't clear depending on the number who pay for a revenue ticket.

3) Space-available upgrades at the airport or gate for purchase on day of departure. This varies highly as to whether airlines offer these and at what price.

4) Operational upgrades. Again, some airlines may not offer them or will seek to give them to passengers of higher status first. These occur when coach is oversold but space exists in business or first class cabins.

In addition, certain travel agencies, called "consolidators" often sell discounted premium tickets when the airline believes it can't sell enough at full-fare but want to get more revenue than by simply selling the upgrade itself.