Auto-save sent PMs
Could you change it so that sent PMs are automatically saved in your Sent Messages folder? Email doesn't require that you specifically save your sent emails and neither does the PM system on literally every other internet bulletin board I've ever used, so I typically forget to click the "Save Copy In Sent Folder" checkbox when I send a PM. It's really annoying. If some people don't like saving copies for whatever reason I guess you could make auto-saving an option in the User Preferences.