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One possible choice would be to pay for a better seat on an airline like United. You can get seating in Economy Plus for an entire year if you buy an annual package for it for $425, or can pay per flight. After a couple of long distance flights, the annual option may make more sense.

It's not business class, but if you work the schedules to get on United airplanes, you get five or six inches more legroom, which is better than nothing.

If you have miles racked up you can also use them to upgrade.

You should try to focus all your flying during the year into one alliance and collect on one airline so you get your miles balance to help with the upgrades.