More and more Americans are buying travel insurance to make sure they aren't left out in the cold when vacation plans go awry. But how do you know if your next trip is worth the extra cost of a policy?

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As with any type of insurance, consider how much you can afford to lose, advises David Lytle, editorial director at Frommers.com.

A $500 plane ticket is probably not worth ensuring but a $10,000 package vacation would be harder to swallow if a natural disaster strikes your destination or the tour company goes belly up.

Also, some travel purchases may be refundable while others aren't. Low-cost plane tickets booked online increasingly come with tight restrictions but hotels will generally allow you to cancel as late as 72 hours in advance, depending on location and season. Cruises and package tours usually require you to pay up front and may not offer refunds.

Generally, you won't have to insure a domestic trip.

If you do decide to buy travel insurance, there are several types from which to choose.

The most common and the most comprehensive is trip protection, which generally covers the entire cost of your trip from transportation to accommodation and anything included in a package vacation, minus the price of the policy.

There is also luggage protection, rental-car liability and medical and evacuation insurance.

Before you buy, check to see what your current insurance policies cover, says Lytle. Your auto policy or credit card may cover you for liability in a rental car and some health insurance will pay for care anywhere in the world. Sometimes you can extend a current policy to cover your travels, too.

Also, take into account any special circumstances. If you have a medical condition or plan to do extreme skiing in the Alps, medical and evacuation coverage could be in order.

One thing you should never do, says Lytle, is purchase insurance from a company with whom you've booked your travel. That includes the extra insurance offered by the rental-car company and especially any coverage offered by a tour operator. It's just a handy way for them to make extra money and if they do go bankrupt, you can just tack that extra sum onto your losses.

Lytle recommends two Web sites for comparing policies: insuremytrip.com and insure.com.

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