I've had pretty good luck using some old milk crates to put a dozen traps in each, then load up as many as my pickup will hold and I make a trip down to the local coin op car wash. The crates keep the traps contained under the high pressure hose force, and our local operation has heated water with a decent soap and rinse time for your quarters. I don't know if its perfect, but it gets every bit of caked on dirt and grime off.
I use speed dip now on most of my traps, and unless I get a trap really shined up from a coon I haven't had much rust issues at all since switching to the speed dip.
I just don't have the facilities set up here for boiling a big batch of traps anymore, so this method works for me.