With the release of xtool v0.9.9, there's certain things that were added to assist you in compressing your games and I've created this little guide to help you use xtool somewhat to its fullest potential.
Before you proceed, first visit this
thread to see if the game you're interesting compressing hasn't been investigated and if you've researched a game want to post your own findings, post them
here.
Topics
Investigation (Simple)
Investigation (Advanced)
Interpreting graphs
Precompressing the game (advanced) [zlib]