I have a 2x4 wood wall with 1/2" regular drywall on both sides, how can I increase my fire rating without installing a new sheetrock on top? Could I increase the fire rating with a paint or plaster or insulation?

Is this for code purposes or just for peace of mind? I don't think ablative coatings are code accepted and would probably cost more than rated drywall. You need 5/8 X rated drywall. If it is code related, they may still not allow a second layer unless it is fire rated(X)so you would still need 5/8.