While Yahoo! Mail Beta is pretty darn sweet, sometimes you just develop a taste for the Classics. However, switching back and forth takes a few clicks and can be more hassle than you want to deal with, especially if you only want to use Yahoo! Mail Classic for a single session or would like to always use a different mail setup between work and home.
The Y! Mail FAQ -- "a collection of the most frequently asked questions from the official Y-Mail User Group, plus many great tips and tools made available by its members" -- has an answer. If you take your regular mail URL, usually of the form:
http://*.*.mail.yahoo.com/*
and replace it with:
http://*.*.mail.yahoo.com/ym/login?ymv=0
you'll be logged into Mail Classic for that session. You can then bookmark that URL for easy access, or if you're like me and use a couple of different accounts that might be in different mail farms, try hitting this bookmarklet that I just wrote -- Classic Mail Switcher* (drag it up to your Bookmarks bar, maybe) -- from any Mail Beta page.
* Note that this'll only work if you're normally on Mail Beta and want to switch back and forth from there. I'm not sure what sort of URL hack you'd use to temporarily flip from Classic to Beta.**
** Yes, it just sends you back to http://mail.yahoo.com if you're at a URL containing both "mail.yahoo.com/" and "ymv=0". Yes, I'm lazy.