Q. If I purchase a ticket a month or two before I am scheduled to fly and then the posted ticket price goes down, can I and how do I get a voucher for the difference?
A. Only a few airlines will give you a voucher without deducting a fee of up to $100.
Currently these are United, Southwest, Alaska, and Jetblue. But this may change. (US Air until recently was also in this group and I've heard an unconfirmed rumor, which I hope is untrue, that United may join the fee chargers).
If only the fare has changed, and you don't change your flight times or dates in either direction, call the airline and ask for the fare difference in the form of a voucher. If you never plan to fly again, you can also get cash back in some cases, but they will deduct a fee.