Main concern is to keep current flowing while switching the motor off. You have to place a high speed flywheel-diode across the DC-motor. Current through the diode is the same as which flows through the motor. When you would use a low-speed diode, very high peak-voltages will occur. Thats why your relay contacts are burning. Also place a large capacitor as buffer for the current. Keep all leads as short as possible between relay (or mosfet), diode and capacitor.
If you need a circuit, please post back.