I have NEVER used a case cracker in my life. What I found helps is pushing on the SCSI port (use the floppy port if you've got a 128K or 512K). Do this while pulling slightly on the case with both hands.

Unscrew the two screws in the lid but don't remove them. They are a pain to put back if removed completely. (I've also seen a few Macs that are missing these, perhaps from a past technician who had bad luck replacing them after a repair or upgrade). By keeping them in the handle shaft of the Mac they will not fall out provided you don't turn the Mac's case back upside down.

This has worked for me on the most stubborn compacts and the ones that have obviously been opened many times before plus all those in between the two extremes.

I've never had any "case joints" damaged by this either, so it seems pretty safe.