theres a thing called coil cleaner.
its a heavy detergent that expands a lot and carries away dirt with it. its sprayed on the coil and left for some minutes.
then clean water sprayed on it.
or you can do what i do. i hang a plastic bag under the indoor so water doesnt drop too much, wrap the electric parts in plastic and then clean it with a pressure washer. blasts away all the dirt. BUT NEED TO BE VERY CAREFULL, as the high pressure water can damage the fins