Airfares can fluctuate even within the span of a few hours, but the direction of holiday travel prices is one-way: up.

Based on early bookings, Travelocity recently reported that the average domestic airfare over Thanksgiving will be about 4 percent higher than last year, to $376, including taxes. Overall, holiday travelers can expect fares to be up from between 3 percent and 10 percent, travel specialists say.

What's behind the increases? Largely supply and demand.

Industry mergers have reduced the number of available seats. Airlines, anticipating fewer passengers because of the weak economy, have cut flights, says Rick Seaney, chief executive of FareCompare.com, a travel website. And, he adds, fuel prices remain too volatile to put any downward pressure on fares.

Besides, with so many people wanting to travel during the holidays, airlines generally don't have to lower prices to attract passengers.

If you haven't started shopping for tickets, get to it now. Set up alerts with the airlines or travel websites such as FareCompare or Airfarewatchdog.com so you can see when fares fall.

And if you see a price that's attractive, grab it instead of waiting to see if it will fall further, travel specialists say. If you hold off too long, you could end up stuck in an uncomfortable middle seat or unable to sit with your travel companions.

Seaney says that starting this week, travelers can expect to see airfares for the Thanksgiving holiday go up each day by an average of $5 per round trip. He advises travelers not to wait beyond the first week of next month to buy a ticket for Thanksgiving.

For departures after Dec. 15, he says, the daily ratcheting up of fares will start the first week of November.

Nonstop flights are pricey but worth the extra money during the winter holidays, travel experts say. Flying nonstop means you don't have to worry about missing a connecting flight or making multiple new arrangements in case of a snowstorm.

Here are other holiday travel tips:

Choose off-peak times In December, consumers spread out their travel days. But for Thanksgiving, most people want flights leaving the Wednesday before the holiday and returning the following Sunday.

Because the Thanksgiving period is in such high demand, you'll pay a premium. For instance, flying the Sunday after Thanksgiving can cost $50 more than usual, Seaney says.

You can save money by flying on less-popular days, says Seaney, who suggests leaving on a Monday or Tuesday and returning on Saturday.

But money isn't the only reason to avoid the peak days.

"That's when lines are out the door at the airport and everyone is stressed," says Christopher Elliott, reader advocate for National Geographic Traveler magazine.

If you really want to avoid the hassle of crowds — and save money — fly on the holiday itself, when planes are about 40 percent empty. That means more room in bins and on armrests.

"If you think about it, you really want to be on the plane that day," Elliott says. "You show up just before dinner begins and someone puts a drink in your hand and you're a happy camper."

Or, Elliott suggests, change the day you celebrate the holiday.