How can I remove old LEGO bricks that are stuck together? Brought my childhood LEGO sets from 1970's and 80's for my kids. The pieces attach much tighter together than sets purchased recently. 

Can something be done to loosen the attachment without damaging the bricks?
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/