Man, that is an age old question. Every time I think about the most effective way to do it, I envision a chain and hook hanging off and old oak tree that I could lift the bow up to about 30-40 degrees and just start blasting it with high pressure. That is obviously not feasible for most of us, but I truly believe letting gravity go to work for you is the most effective way. It seems to me that all other ways are just inefficient in doing a COMPLETE job.
High pressure scares me as I don't want to loosen up the backing or heaven forbid cut a tear from the pressure, you'd have to use a pretty wide nozzle fitting on the pressure sprayer to make sure that didn't happen.