the best way to clean them is by using a pressure washer.
Cleaning by hand just doesnt do a good job really. You cant get into all the nooks and crannies.
Cleaning with wfp is ok, it does the job, but it is hard work! Especially cleaning all the soffets.(i think thats how you spell it) you really need to let the water soak in first though. Or ideally you could do it in the rain, that will soften all the dirt up before hand, making it quicker to get off.