1.Clean the parts piece by piece before you put them on, or tear it all down and clean it piece by piece.
2. A not so great idea is tape up or put plastic around everything you don't/can't get wet and degrease and powerwash it. With the second option, your risking breaking brittle pieces, cutting wires, rust, bending fins on the IC, condensor and radiator. I have tried both ways, the first option is time consuming but well worth it and it's safer. I always clean my engine or whatever it is piece by piece if I need to, before I put it in, that way the gaskets can be changed as well.