Currently, users only get a discount to transfer between Metrorail and Metrobus. Instead, make that transfer free - the rider will only need to pay the difference in fare.
Free transfers will help improve the value of the entire transit network and work to better integrate the entire transit network. Free transfers will help provide value for riders and allow more flexibility for their trips. The direct cost of implementing the change should be relatively low as the SmarTrip system already processes transfers; however this does not account for potential lost fare revenue. However, experience in other cities has shown that free transfers don't decrease revenue much, but dramatically increase ridership.
For example, consider the results in New York when they implemented free bus-rail and rail-bus transfers in the late 1990s: http://www.schallerconsult.com/pub/metrocrd.htm