How can I remove old LEGO bricks that are stuck together?
Try using this brick separator tool


Source: https://shop.lego.com/en-US/Brick-Separator-630

A simpler version used to come with older sets. Some of the images demonstrate how the tool may be used with different kinds of pieces.


Source: http://www.philohome.com/remover/remover1.htm

Two brick separators may be used for maximum separating efficiency for certain types of stuck brick situations.

For example, when two plates are stuck together:



Alternatively, using it as a lever to remove tiles



Source: http://thebrickblogger.com/2010/11/lego-brick-separator/