I often find that some of the more detailed information I need to complete a task is contained within an email that is in my email. If the extra information is brief then I will copy and paste it into the notes field for the task. If, however, there is a lot of information in the email (links, pictures, acres of text) then it can be much more efficient to link the two. Simply open up the relevant email, copy the URL from the address bar at the top, and paste it into the URL field on you RTM task. Applying an "RTM" label to those emails in Gmail also serves as a handy double check that you've logged all the tasks you want in your email.