In reply to StevoNorthernClimber:
If the old glue is still there but just a bit lumpy and no longer evenly spread, it's possible to get good results by simply ironing it flat again. Apply the iron directly to the glue and once it's hot it's quite amenable to being re-spread across the skin, either by pushing it around with the iron or with a scraper. You can clean the iron afterwards with white fuel.
However if the old glue is mostly not present or full of grit & bits of heather then DIY re-gluing is not as hard as it might sound. I started my first re-gluing fully expecting an afternoon of cursing then having to be rescued from the fly-paper trap by laughing friends but it was actually fine. The iron-on glue strips (http://www.facewest.co.uk/Coll-Tex-Adhesive-Transfer-Tape.html) are pretty easy to use and I'd definitely go for those if you're doing the whole skin.
The usual method for removing the old glue, as HeMa describes, is to heat it until it will soak into something absorbent but you can save quite a bit of time by heating it directly with your iron then scraping it off. You can then soak up the last remaining glue with rags.