How do I enable text message archives?
To enable text message archives:

Click the Messenger menu.
Select Preferences.
Click Archive.
Select one of the three options.
"Save all my messages" saves every IM conversation, even when you sign out. This option is selected by default when you install Yahoo! Messenger.
"Save all my messages, but clear them each time I sign out" is a great choice for a shared public computer. During the time you're signed in, you view your recent conversations, but once you sign out, all of those messages will be deleted.
"No, do not save any of my messages" means archiving is OFF. No items will be saved.
Click Apply, then OK at the bottom of the window.