Quoted from jrpinball:Take the bottom board outside, put a tub or shallow pan under it, lean it up against something, and hose it down with aerosol brake cleaner.
About two cans worth will clean the grimiest board, and will clean the wiring too. Let it air dry thoroughly, or use an air gun to speed up the process. The labels and relay stickers may not fare very well, but they won't in a water bath either.
Edit: I rewound about a year back, and see that this method has been proposed already.
This is what I'll be doing to my Spanish Eyes. Both the bottom board and the head board. There's grime, grease, and gunk in there. I was thinking of going the simple green, scrub, and hose route, but I think brake cleaner will be much more effective. I'll spend the money on 4 bottles of brake cleaner if it'll save me 75% of the time.