If you bought through a travel agent, ask the agent to monitor fares for you. Otherwise, try Travelocity's Fare Watcher feature, which sends you an e-mail if your seat goes on sale. They don't anymore because airlines have cracked down on the practice. But I wouldn't suggest anyone try hidden city. It's really risky.
Airlines say that hidden-city tickets violate their fare rules.
To prevent you from using them, many now program their computers to cancel your reservation when you don't use the first leg of your return trip. When you show up for the second leg, you may be stuck buying a ticket home at the walk-up fare. That, too, is against the rules, but it would be tough to catch. The same is true if you find you have to buy a one-way ticket. The round-trip is certain to be cheaper, and the airline can't penalize you if you throw away the second half of the ticket.