Renting a reliable vacation home isn't easy, and not just because there are a seemingly endless number of rental resources to turn to — everything from local sites that list a few condos to big listing services like HomeAway.com or VRBO.com.

An estimated 1 million vacation rentals are in the United States, roughly half of which are available to the general public, according to numbers from the National Association of Realtors and FlipKey.com. A recent PhoCusWright study estimated that vacation rentals are a $24.3 billion-a-year market, and a Ypartnership survey suggested that interest in renting a home is on the rise, with 14 percent of leisure travelers saying they stayed in one in 2009, up from 11 percent a year earlier.

"The consistent challenge we hear from travelers is predictability," says TJ Mahony, the chief executive of FlipKey.com, a vacation rental site. "People tend to know what they are going to get from a hotel but can have anxiety over the quality of a vacation rental."

I asked experts to identify the biggest challenges when renting a home. Here's what they told me:

1. Knowing what you want. "You are going to pay more for certain things. Decide what is really important to you," says Teresa Bell Kindred, a blogger and frequent home renter. Her family loves the ocean, so they pay more to be right on the beach. If you don't mind being a few blocks away from the water, you can save money.

2. Timing your purchase. If you're attending a special event, like a sports tournament or festival, book as early as possible. "Vacation rentals are more scarce than hotels, and great vacation properties are even scarcer," says Chris Brusznicki, the chief executive of GamedayHousing.com, a site that specializes in rentals for sports events. But if you're flexible, run down the clock. One terrific new site that allows you to bid on "last-minute" vacation rentals is called PackLate.

3. Finding out what you're renting. "Do your homework," says Sylvia Guarino, who owns a rental home on Sanibel Island, Fla. (one of my favorite places) and is a member of Second Porch, a Facebook application that connects vacation rental owners and vacation guests. "Vacation rental guests sometimes get too focused on getting a deal and not focused enough on getting the information that they need, or authenticating the property." How do you authenticate a property? Look at every piece of information available to you, including the owner's site, the vacation rental site, online reviews and what you find on social networking sites such as Facebook. Be sure the property is in good shape, is as represented and isn't in foreclosure.

4. Finding out who you're renting from. "Research the owner, not just the home," says Kelly Hayes-Raitt, who owns a rental property in Santa Monica, Calif. She includes a link to her website and biography in every correspondence with potential guests, "not just so they realize they are supporting my work with refugees, but so they learn they are dealing with someone who has been active in her community for 30 years and is not likely to cheat them," she says. Not every owner is as forthcoming as Hayes-Raitt, but if someone is reclusive, that might be a warning sign.

5. Determining whether the rental is part of an association or destination. That can make a big difference, according to Jon Ervin, a spokesman for the Cottage Rental Agency in Seaside, Fla. "Imagine you rent from Mr. and Mrs. Jones — nice enough people, but what if your air conditioning quits or some other concern arises?" he asks. "You most likely are going to have to work through the issue for your entire stay." If your rental is part of an association, someone will be on call to help.

6. Becoming a power user. For example, VRBO.com allows you to filter search results by area, bedrooms and the number of people it can sleep. That helped Kellie Pelletier find a vacation rental in Charleston, S.C., for her family. VRBO also shows which homes are available on your preferred dates. "I didn't waste my time researching and contacting houses that were already booked for my week," she says.

7. Avoiding group-think. Extended families should be broken up into smaller housing units, such as adjoining condos, as opposed to fitting them into one house. It gives everyone more privacy and is easier on the finances, because you don't have to argue over the bill at the end.

8. Reviewing the contract very carefully. You're not checking into a hotel; this is more like renting an apartment. Watch for contract language, such as cleaning options. Frank Discala, who owns a rental property in Nantucket Town, Mass., gives his tenants two options: either clean up after themselves, "or they can leave the place without cleaning up and lose their $500 deposit," he says. "Ouch! No one has ever taken that option."

9. Staying flexible. Remember, you're renting an apartment or house — not a room in a hotel. "Some things may go wrong," warns Pauline Kenny, a vacation rental expert who runs a site called Slow Europe. "The plumbing may stop working, the kitchen stove runs out of gas. Some things may not be perfect — you bump your head repeatedly on that low doorway, the couch is orange, the parking space is almost impossible to get into." That's life in a vacation rental. "Suck it up and live with it," she says.

Christopher Elliott is the ombudsman for National Geographic Traveler magazine. You can read more travel tips on his blog, elliott.org, or e-mail him at celliott@ngs.org.