Great question. The first step to do is to always save all your file as a maya Ascii. Why? this way it encrypts everything using maya's mel language which can be easily edited in Note pad. Once you do this, when you save something in Maya 8, and want to open it in Maya 7, open the Maya ascii file with notepad and replace everything that says Maya 8.0 to 7.0 , it should be within the first few lines. After you replace everything with 7.0 , save it and now you can open it with Maya 7.0

This doesn't apply when you go up in versions, only when you downgrade.