(Fortune Magazine) -- Next to expense accounts and free coffee, in-house travel agents have to be the best invention of the modern corporation. The good ones know how to find the flights that have better chances for upgrades, or search for the fares that allow much more flexibility without much more dough.

For those of us without an agent at our disposal, however, it's a headache. Nailing down frequent-flier tickets or upgrades often requires multiple phone calls, numerous web searches, and too many compromises.

But there's a better way: Check out expertflyer.com, a website that essentially gives you a peek inside nearly every airline's stockroom. That's important because on a given flight, there are dozens of different fares and ways to qualify for an upgrade.

Often if you book online you see only the lowest price or the full-fare unrestricted ticket. But some airlines have cheaper unpublicized fares that offer greater flexibility and allow easier upgrades. The site, which starts at $4.99 a month, is aimed at travel pros, but a new "consumer version" is on the way this fall.

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