Smoked up Fireplace Doors

We bought a new wood air heating/fireplace system. It is gorgeous. We love it however we paid extra for glass doors.So we could sit and watch fire burn. The problem is you can't see thru them as they are always blacked up! Other than cleaning them with oven cleaner every day so we can watch the fire burn don't know what else to do. Any body know of something that can maybe be sprayed on them or something that will keep them from getting so blacked up. Would appreciate any suggestions.