NEW YORK, March 11 (Reuters) - Airlines and travel agents, their relationship strained in recent years, plan a joint promotion on Monday to boost air travel, exactly six months after the Sept. 11 attacks on New York and Washington shook public confidence in flying. Travel agent associations are teaming up with Delta Air Lines (DAL) and Southwest Airlines (LUV) in a public relations offensive to inform the flying public about aviation security changes, encourage travel and tourism, and promote travel agency services. Travel agencies are under competitive pressure from Internet-based services that generate billions of dollars in sales and include sites run or supported by most of the biggest airlines. Most major carriers have cut the commissions they pay to travel agents, sparking bitter controversy and raising the stakes for traditional booking agents. The Sept. 11 attacks, coupled with recession, threw the U.S. travel and tourism industry into dramatic decline. After months of staggering losses, key players in these industries are bolstering alliances to help win back business and regain their financial footing. Air travel has recovered slowly since October. Passenger volume for the big U.S. carriers was off 12.8 percent in January, the latest figures available, compared with last year, but security is still a major concern of travelers. The initiative to be unveiled at a news conference in New York on Monday, "The Flight Plan for America," features a Web-based program for travel industry professionals and a brochure for travel agency clients, designed to help passengers prepare for new security checks and avoid airport hassles. Airline passengers are now being asked to show up between one and two hours before their flights to allow time for longer check-in and security procedures. The airline and travel groups have recruited former astronaut James Lovell, who survived probably the most perilous space flight in history while commanding Apollo 13, to pitch their message to the public. "Travel is down now, in part, because of perceptions that new airport security measures are too much of a hassle or are inadequate," said Kathleen Argiropoulos, the chairwoman of the Flight Plan for America Committee. A goal of the group is to inform travelers about efforts to improve airline security. The government has taken over responsibility for passenger and baggage screening from the airlines, and is in the process of creating a work force of 30,000 screeners to be deployed at more than 400 airports by November. The Transportation Department has also overhauled security requirements for airport workers, especially those with access to gates, ramps and aircraft, and has cracked down on those who fraudulently received airport security credentials. A survey released in conjunction with Monday's air travel promotion found that 38 percent of 500 business and leisure travelers polled cited the economy as their chief concern affecting travel. Thirty one percent cited security-related issues, split between concerns about the level of security and problems triggered by new security measures. Additionally, 8 percent expressed a fear of flying.