Ah, good old hidden city ticketing. For reasons beyond the scope of this article, it's apparent that airlines price certain city pairs much more cheaply than others. A lot of it has to do with demand as a lot of smaller airports tend to have somewhat higher pricing. Also, distance doesn't seem to have a strict relationship to price when dealing with hidden city ticketing. For instance, I've seen LAX to Chicago O'Hare go for $200 although LAX to Salt Lake City (much closer) was going for closer to $450 for the same weekend.
Whatever the case, savvy travelers will often fly to these cheap cities but have a "stop" at the city they really want to go to. So for instance, you're in LA again and you see a really cheap US Airways flight to Boston. However, you want to go to Philadelphia. Being well informed, you know that Philly is a major hub of US Airways. You proceed to book the flight to Boston one way, then you fly nonstop from LA to Philadephia. This is the most important part of the procedure: you must essentially "throw away" your last segment from Philadelphia to Boston and get off the plane in the destination you really want to be.
A couple of caveats:
1. If you check any luggage, your bags will be shipped to the final destination on paper. Hence, you must only have carry-on luggage to make this work.
2. This is usually good only for for one-way trips. This is due to the fact that if you miss a segment of your flights on a roundtrip itinerary, the rest of your flights will also be cancelled.
3. Airlines know about these tricks, but they don't like them very much. Of course, you're not breaking any laws by employing hidden city ticketing, but I'd advise not doing it too often at the risk of incurring the airline's wrath.
I've been using hidden city ticketing for many years and I've usually saved several hundred dollars off of my flights each time I've done it. A great way to search for these deals is through ITA Matrix, an online flight search engine. Scott from Hackmytrip.com has a great primer on it here. Good luck!