If you want OS X during boot and have OS 9 selected as the startup disk, hold the 'X' key while booting. But if you want OS 9 and you have OS X selected as the startup disk, holding '9' won't get you anywhere. But hold Command-S instead (which launches in single user mode), and then type:
bless -folder9 "/System Folder" -use9
reboot
This will restart your computer into OS 9 without it having to fully start up on X. To make it even easier, make a script that does this, put it in the root of your hard drive and chmod it so it is executable. However the default shell is not bash (anyone know how to change it so it is?).
Mac OS X Hints
http://hints.macworld.com/article.php?story=20041122025940469