I use lots of sinew so thought I'd offer some thoughts. As for cleaning the sinew, I'd agree with hillbilly in that once it's processed (all meat, fat and membrane has been scraped off, and it appears a clean white or, at worst, a very translucent beige) there is no further cleaning necessary. I wouldn't even bother with the sinew of a domestic animal, as they're too fatty.
As for preparing the back of the bow, I usually just wash several times with dishwashing detergent and water as hot as you can stand. The hot water itself helps take the oils into solution and drag them out of the wood. I always add a layer of hide glue before laying sinew, but I've heard others say this isn't needed. I never score the bow's back, leaving it smooth, which again seems at odds with what many folks do. I've never had a problem with sinew pulling free either, though, leading me to believe that scoring isn't necessary as long as the bow's back was adequately degreased.
All the best,
-Eric