Perhaps many people know this, but I sure didn't...
I was frustrated by Mail automatically including the original attachment of the mail to which I was replying.
The option to include/discard an attachment when replying is not in Mail's preferences. Rather, it can be found under the menu Edit -> Attachments (along with an option to always send Windows-friendly attachments). This menu item allows you to toggle the option on or off. Note that you can't make this change in edit mode on a message; you must be in the main viewer window.
[robg adds: I'm pretty sure this option is disabled in a stock Mail install, but just in case someone's having the problem, here's the fix ... and why these settings aren't in the Preferences section of Mail makes no sense to me at all.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050829024124221