Lego makes two 'removal' tools, those are handy, then, I have a very sharp naked 4" blade that will take care of the hardest cases, baseplates will flex the tiniest bit and that will usually give you enough space to flip the top part off. Finally, a regular plate pushed at a 45 degree angle into the space between two other plates will separate the two without damage to any of the parts if you do it 'just so'.
