Booking expensive flights, hotel, rental car and standard travel items could be alleviated with incentives for government employees to save. Rather than take a flight from D.C. to Philadelphia, take a train. Rather than stay at a hotel, stay with friends or family in the city. And rather than using a rental car, take public transportation in the location if feasible.
All of these come at a certain inconvenience, yet could save a great deal of money. To incentivize this practice, allow employees to receive a fraction of the difference that would be expected in a normal trip.
For instance, if a business trip to Philadelphia normally would cost $700 for a roundtrip flight through the travel agency and two nights at a hotel totaling $400, but the employee takes a roundtrip train ride for $200 and stays with family, this employee has saved the government $900. To compensate, the employee could (for example) receive 1/3 of the cost difference. The employee has a $300 incentive to take a longer train ride and stay with family and the government still saves $600.
This may be limited in the amount of travel details it could be applied to, but certainly for big expenditures where alternatives to flights and lodging are present, this could save the government and taxpayers extensively.