The best way to clean the frame of your mountain bike is to use a mild detergent and a brush to remove any dirt, mud, or other debris. Be sure to rinse the frame thoroughly with water to prevent any soap residue from remaining on the frame. You may also want to use a specialized frame cleaner to make sure the frame is thoroughly cleaned. Additionally, you may want to consider waxing the frame to protect it from the elements and to give it a glossy shine.