Pack light or pay more. That's the message for your attendees starting this month when five U.S. airlines begin charging extra for checking a second piece of luggage.

In early February, United Airlines announced it would charge $50 roundtrip for a second piece of checked baggage starting May 5. US Airways followed suit three weeks later, and Continental, Northwest, and Delta airlines soon joined ranks. With airlines under pressure from skyrocketing fuel prices, some airline analysts believe American Airlines will be next.

The new fees apply to tickets purchased on or after the date the new policies were announced.

In United's case, the new checked bag fees do not extend to international travel or to customers enrolled in its top-tier frequent-flyer programs.