This may be an obvious one for the power users, but still:
If you only plan to use OSX you can just do this, otherwise you may want to have a separate OS9 install for classic and one for booting into natively.
Don't install 9.1 taking the defaults. Choose a custom installation and rmove all the extras (text to speech, IE/Outlook, etc) basically only installing "Base" and the default networking. You may also want Quicktime and the standard fonts. Beyond that you don' t need the rest of OS9 to run apps under OSX, as OS X does much of the work for OS 9.
By removing all the extra I've reduced my Classic launch time from a minute and twenty seconds to twenty-two seconds.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2001041123533995