While a razor blade can done safely, if they're "cracked" as you say, that will be a VERY tedious job.  Another method is using a heat gun to help soften the glue and allow the peeling/pulling to go pretty quickly.  But again, with the "cracking", you're constantly starting over.  For this case, the best tool I know of is made by 3M.  I call it an "eraser" tool - it's about 10, 6" rubber discs all stacked together.  Each one is very thin - think about what 10, flexible, CD's would be like.  They are clamped together in the center and the clamp/post is inserted into a high speed (a couple thousand RPM's is needed) drill.  It works like MAGIC.  Google it - you'll like it!  Acetone will clean up any remaining glue residue.