Did you know that you can find coach tickets that come with an instant upgrade to First Class? Search for these discounted First class tickets at farecompare.com or at some airline sites:

American Airlines offers this feature under "search by price and schedule" and look for the "instant upgrade" column in your search results)

USAirways offers this feature in its advanced search under "instant upgrade" fares.

Farecompare also has a guide to finding Q-UP / Y-UP / M-UP fares here.