Here are three ideas to give an incentive to spend less on travel:
1) You can pay for the flight as usual (through a travel contractor) or you can make your own arrangements and be reimbursed 50% of the ticket price.
2) You can have the government pay for your hotel (pay using a government credit card aka travel card) or you can be reimbursed 50% of the per diem rate for the hotel if you find your own lodging.
3) You can have the governemnt pay for the actual cost of all meals (government credit card) or receive 50% of the per diem rate for M & IE.
If you choose to use a government credit card to pay for your per diem, but still spend less than 50%, you are reimbursed the difference.
This idea would be fairly painless to those who love the current travel system. The only thing it would stop is making a profit off of a high per diem M & IE, but M & IE is intended to feed you, not give you a travel bonus.