Airlines in the U.S. are expected to have their best spring in nearly a decade, continuing the recent trend of improvement within the industry, but that doesn't mean flyers should expect a break.

Newark Liberty International Airport (Chris Hondros, Getty Images)

Experts suggest the add-on charges and extra fees that airlines have introduced to make ends meet aren't going anywhere. If they do, they're going up.

According to Douglas Kidd, executive director of the National Association of Airline Passengers, special fees have helped airlines make "substantial amounts of money" above ticket prices.

"They're not about to give up a revenue stream," Kidd said.

To keep the profits coming, airlines have been engaging in a practice known as capacity discipline, letting demand drive the number of seats available to a certain destination. Instead of adding another flight to accommodate customers, the goal is to pack one flight as much as possible, and perhaps get as much money as possible from each passenger.

"They've been running at, recently, load factors above 85 percent," Kidd said. "Whereas maybe 10 to 15 years ago, if a plane was half-full, the airlines would consider themselves lucky."

The industry has been performing exponentially well in recent years when compared to the middle-late segment of last decade.

The trade group Airlines for America predicted passenger volumes in March and April will rise to their highest level in seven years. The group said airlines will respond by increasing the number of seats by 3 percent.

Carol Piscitelli, owner of Gemini Travel Agency in Bloomfield, said fees for baggage, food and specific seats will continue as long as flyers are willing to pay them.

"I am mystified," she said. "People will spend $600 just in seat numbers for a family of five."

Piscitelli said in some cases, "you might as well go first class" once fees are added into the equation.

She's a firm believer in searching for airline fares on Tuesdays and Wednesdays in order to get the best prices. Also, it doesn't always help to buy tickets way in advance.

"You should always call a travel agent, even if you have to pay a service fee," Piscitelli said. "Travel agents do not make commission on airline tickets."