NEW YORK - On top of the bag fees and other charges, families traveling this summer may have to pay extra just to sit next to one another.

Airlines are reserving a growing number of seats for elite customers or those willing to shell out more money. These seats often, but not always, come with a little extra legroom. The catch: setting these seats aside leaves fewer places for other passengers to sit without paying extra.

That means mom might end up in row 20, dad in row 23 and junior sitting all the way back in row 30, regardless of age. Airlines say their gate agents try to help family members without adjacent seats sit together, especially people flying with small children. Yet there is no guarantee things will work out.

Often, the only way to ensure that the family sits together is to book well in advance, or pay $25 or more, per seat, each way.

Summer is an especially hard time to keep families together. Last July, U.S. airlines sold a record 87.8 percent of seats on domestic flights, according to the Bureau of Transportation Statics. And that figure doesn't include seats occupied by airline employees flying for free. In other words: virtually every seat was taken.

Airlines are predicting that even more passengers will step aboard planes this year: 222 million passengers between June 1 and Aug. 31. That's 2.4 million passengers a day or 4.5 percent more than last summer.

But don't fret. There are still a few things fliers can do between now and takeoff to snag seats together without paying extra:

- Confirm your seat assignments online with the airline now. Sometimes plane types are switched between booking and departure - say from a 737 to a 757 - meaning there's a whole new seat layout. And if you booked through Expedia, Orbitz, Priceline or other third-party travel agents, your assignment might not have been recorded correctly with the airline.

- Set up alerts for seat openings. ExpertFlyer.com offers free notifications when a window or aisle seat becomes vacant for up to one flight at a time. For 99 cents, it sends an email if two adjacent seats become available. The service is available for Alaska Airlines, American Airlines, JetBlue Airways, United Airlines and Virgin America but not for Delta Air Lines and some smaller carriers.

- Check your airline's website five days before the trip. That's when some elite fliers are upgraded to first class, freeing up their coach seats. Another wave of upgrades occurs every 24 to 48 hours.

- Check in 24 hours in advance when airlines start releasing more seats. If connecting, see if seats have opened up 24 hours before the second flight departs.

- Keep looking for new seats. Even after checking in, seats can be changed at airport kiosks and on some airlines' mobile applications.

- Ask at check-in. Agents can sometimes put families in seats held just prior to departure for disabled passengers. And ask more than once. Gate agents and flight attendants can sometimes seek volunteers to swap seats.

- If flying Southwest Airlines set an alarm for exactly 24 hours before departure and check in online. The airline lets passengers pick their own seat at boarding. The earlier you check-in, the quicker you board the plane.

- If all else fails, offer nearby passengers candy or a drink to switch seats. Seriously, bartering works.